Serological response in RT-PCR confirmed H1N1-2009 influenza a by hemagglutination inhibition and virus neutralization assays: an observational study

Dynamics of Anti-influenza Mucosal IgA Over a Season in a Cohort of Individuals Living or Working in a Long-term Care Facility

BACKGROUND

Serological surveys are used to ascertain influenza infection and immunity, but evidence for the utility of mucosal immunoglobulin A (IgA) as a correlate of infection or protection is limited.

METHODS

We performed influenza-like illness (ILI) surveillance on 220 individuals living or working in a retirement community in Gainesville, Florida from January to May 2018, and took pre- and postseason nasal samples of 11 individuals with polymerase chain reaction (PCR)-confirmed influenza infection and 60 randomly selected controls. Mucosal IgA against 10 strains of influenza was measured from nasal samples.

RESULTS

Overall, 28.2% and 11.3% of individuals experienced a 2-fold and 4-fold rise, respectively, in mucosal IgA to at least 1 influenza strain. Individuals with PCR-confirmed influenza A had significantly lower levels of preseason IgA to influenza A. Influenza-associated respiratory illness was associated with a higher rise in mucosal IgA to influenza strains of the same subtype, and H3N2-associated respiratory illness was associated with a higher rise in mucosal IgA to other influenza A strains.

CONCLUSIONS

By comparing individuals with and without influenza illness, we demonstrated that mucosal IgA is a correlate of influenza infection. There was evidence for cross-reactivity in mucosal IgA across influenza A subtypes.

Natural influenza infection produces a greater diversity of humoral responses than vaccination in immunosuppressed transplant recipients

The humoral immune response to influenza virus infection is complex and may be different compared to the antibody response elicited by vaccination. We analyzed the breadth of IgG and IgA responses in solid organ transplant (SOT) recipients to a diverse collection of 86 influenza antigens elicited by natural influenza A virus (IAV) infection or by vaccination. Antibody levels were quantified using a custom antigen microarray. A total of 120 patients were included: 80 IAV infected (40 A/H1N1 and 40 A/H3N2) and 40 vaccinated. Based on hierarchical clustering analysis, infection with either H1N1 or H3N2 virus showed a more diverse antibody response compared to vaccination. Similarly, H1N1-infected individuals showed a significant IgG response to 27.9% of array antigens and H3N2-infected patients to 43.0% of antigens, whereas vaccination elicited a less broad immune response (7.0% of antigens). Immune responses were not exclusively targeting influenza hemagglutinin (HA) proteins but were also directed against conserved influenza antigens. Serum IgA responses followed a similar profile. This study provides novel data on the breadth of antibody responses to influenza. We also found that the diversity of response is greater in influenza-infected rather than vaccinated patients, providing a potential mechanistic rationale for suboptimal vaccine efficacy in this population.

Antibody Responsiveness to Influenza: What Drives It?

The induction of a specific antibody response has long been accepted as a serological hallmark of recent infection or antigen exposure. Much of our understanding of the influenza antibody response has been derived from studying antibodies that target the hemagglutinin (HA) protein. However, growing evidence points to limitations associated with this approach. In this review, we aim to highlight the issue of antibody non-responsiveness after influenza virus infection and vaccination. We will then provide an overview of the major factors known to influence antibody responsiveness to influenza after infection and vaccination. We discuss the biological factors such as age, sex, influence of prior immunity, genetics, and some chronic infections that may affect the induction of influenza antibody responses. We also discuss the technical factors, such as assay choices, strain variations, and viral properties that may influence the sensitivity of the assays used to measure influenza antibodies. Understanding these factors will hopefully provide a more comprehensive picture of what influenza immunogenicity and protection means, which will be important in our effort to improve influenza vaccines.

Activated CD4+ T cells and CD14hiCD16+ monocytes correlate with antibody response following influenza virus infection in humans

The failure to mount an antibody response following viral infection or seroconversion failure is a largely underappreciated and poorly understood phenomenon. Here, we identified immunologic markers associated with robust antibody responses after influenza virus infection in two independent human cohorts, SHIVERS and FLU09, based in Auckland, New Zealand and Memphis, Tennessee, USA, respectively. In the SHIVERS cohort, seroconversion significantly associates with (1) hospitalization, (2) greater numbers of proliferating, activated CD4⁺ T cells, but not CD8⁺ T cells, in the periphery during the acute phase of illness, and (3) fewer inflammatory monocytes (CD14^hiCD16⁺) by convalescence. In the FLU09 cohort, fewer CD14^hiCD16⁺ monocytes during early illness in the nasal mucosa were also associated with the generation of influenza-specific mucosal immunoglobulin A (IgA) and IgG antibodies. Our study demonstrates that seroconversion failure after infection is a definable immunological phenomenon, associated with quantifiable cellular markers that can be used to improve diagnostics, vaccine efficacy, and epidemiologic efforts.

Post-infection seroconversion is associated with severity of influenza virus infection

Seroconverters have early proliferation and activation of CD4⁺ T cells

CD8⁺ T cells are unaffected

CD14^hiCD16⁺ monocytes in the blood and nasal mucosa is associated with antibody response

Antibody and Cell-Mediated Immune Responses Are Correlates of Protection against Influenza Infection in Vaccinated Older Adults

Despite efforts to design better vaccines for older adults, the risk for serious complications of influenza remains disproportionately high. Identifying correlates of vaccine effectiveness and understanding the heterogeneity of health outcomes in older adults are key to the vaccine development pipeline. We sought correlates of protection against laboratory-confirmed influenza illness (LCII) in a 4-year randomized trial of standard versus high-dose influenza vaccination of adults 65 years and older. To this end, we quantified serum hemagglutination-inhibition (HAI) titers and interferon-gamma (IFNγ) and interleukin-10 (IL-10) secretion by virus-challenged peripheral blood mononuclear cells. Of the 608 participants included, 26 developed either A/H3N2-(n = 17) or B-LCII (n = 9) at 10-20 weeks post-vaccination. Antibody titres for A/H3N2 at 4-weeks post-vaccination were significantly associated with protection against LCII, where every 1-standard deviation increase reduced the odds of A/H3N2-LCII by 53%. Although B-titres did not correlate with protection against B-LCII, the fold-increase in IFNγ:IL-10 ratios from pre- to 4-weeks post-vaccination was significantly associated with protection against B-LCII, where every 1-standard deviation increase reduced the odds by 71%. Our results suggest that both antibody and cell-mediated immune measures are valuable and potentially complementary correlates of protection against LCII in vaccinated older adults, although this may depend on the viral type causing infection.

A randomised trial of two 2-dose influenza vaccination strategies for patients following autologous haematopoietic stem cell transplantation

BACKGROUND

Seroprotection and seroconversion rates are not well understood for 2-dose inactivated influenza vaccination (IIV) schedules in autologous haematopoietic stem cell transplantation (autoHCT) patients.

MATERIALS/METHODS

A randomised single-blind controlled trial of IIV in autoHCT patients in their first year post-transplant was conducted. Patients were randomised 1:1 to high dose (HD) IIV followed by standard dose (SD) vaccine (HD-SD arm) or two SD vaccines (SD-SD arm), 4 weeks apart. Haemagglutination inhibition (HI) assay for IIV strains was performed at baseline, 1, 2 and 6 months post-first dose. Evaluable primary outcomes were seroprotection (HI titre ≥40) and seroconversion (4-fold titre rise) rates and secondary outcomes: geometric mean titres (GMT), GMT ratios (GMR), adverse events, influenza-like-illness (ILI) and laboratory-confirmed influenza (LCI) rates and factors associated with seroconversion.

RESULTS

Sixty-eight patients were enrolled (34 per arm) with median age of 61.5 years, majority male (68%) with myeloma (68%). Median time from autoHCT to vaccination was 2.3 months. For HD-SD and SD-SD arms, percentage of patients achieving seroprotection was 75.8% and 79.4% for H1N1, 84.9% and 88.2% for H3N2 (all p>0.05) and 78.8% and 97.1% for influenza-B/Yamagata (p=0.03), respectively. Seroconversion rates, GMT and GMR, number of ILI or LCIs were not significantly different between arms. Adverse event rates were similar. Receipt of concurrent cancer therapy was independently associated with higher odds of seroconversion (OR 4.3, 95% CI 1.2-14.9, p=0.02).

CONCLUSIONS

High seroprotection and seroconversion rates against all influenza strains can be achieved with vaccination as early as 2 months post-autoHCT with either two-dose vaccine schedules.

Hemagglutinin and Neuraminidase Antibodies Are Induced in an Age- and Subtype-Dependent Manner after Influenza Virus Infection

Data on the immunologic responses to neuraminidase (NA) is lacking compared to what is available on hemagglutinin (HA) responses, despite growing evidence that NA immunity can be protective and broadly cross-reactive. Understanding these NA responses during natural infection is key to exploiting these properties for improving influenza vaccines. Using two community-acquired influenza cohorts, we showed that the induction of both HA and NA antibodies after infection is influenced by age and subtypes. Such response dynamics suggest the influence of immunological memory, and understanding how this process is regulated will be critical to any vaccine effort targeting NA immunity.

Despite evidence that antibodies targeting the influenza virus neuraminidase (NA) protein can be protective and are broadly cross-reactive, the immune response to NA during infection is poorly understood compared to the response to hemagglutinin (HA) protein. As such, we compared the antibody profile to HA and NA in two naturally infected human cohorts in Auckland, New Zealand: (i) a serosurvey cohort, consisting of pre- and post-influenza season sera from PCR-confirmed influenza cases (n = 50), and (ii) an immunology cohort, consisting of paired sera collected after PCR-confirmation of infection (n = 94). The induction of both HA and NA antibodies in these cohorts was influenced by age and subtype. Seroconversion to HA was more frequent in those <20 years old (yo) for influenza A (serosurvey, P = 0.01; immunology, P = 0.02) but not influenza B virus infection. Seroconversion to NA was not influenced by age or virus type. Adults ≥20 yo infected with influenza A viruses were more likely to show NA-only seroconversion compared to children (56% versus 14% [5 to 19 yo] and 0% [0 to 4 yo], respectively). Conversely, children infected with influenza B viruses were more likely than adults to show NA-only seroconversion (88% [0 to 4 yo] and 75% [5 to 19 yo] versus 40% [≥20 yo]). These data indicate a potential role for immunological memory in the dynamics of HA and NA antibody responses. A better mechanistic understanding of this phenomenon will be critical for any future vaccines aimed at eliciting NA immunity.

IMPORTANCE Data on the immunologic responses to neuraminidase (NA) is lacking compared to what is available on hemagglutinin (HA) responses, despite growing evidence that NA immunity can be protective and broadly cross-reactive. Understanding these NA responses during natural infection is key to exploiting these properties for improving influenza vaccines. Using two community-acquired influenza cohorts, we showed that the induction of both HA and NA antibodies after infection is influenced by age and subtypes. Such response dynamics suggest the influence of immunological memory, and understanding how this process is regulated will be critical to any vaccine effort targeting NA immunity.

Rapid and simple colorimetric detection of multiple influenza viruses infecting humans using a reverse transcriptional loop-mediated isothermal amplification (RT-LAMP) diagnostic platform

Background

In addition to seasonal influenza viruses recently circulating in humans, avian influenza viruses (AIVs) of H5N1, H5N6 and H7N9 subtypes have also emerged and demonstrated human infection abilities with high mortality rates. Although influenza viral infections are usually diagnosed using viral isolation and serological/molecular analyses, the cost, accessibility, and availability of these methods may limit their utility in various settings. The objective of this study was to develop and optimized a multiplex detection system for most influenza viruses currently infecting humans.

Methods

We developed and optimized a multiplex detection system for most influenza viruses currently infecting humans including two type B (both Victoria lineages and Yamagata lineages), H1N1, H3N2, H5N1, H5N6, and H7N9 using Reverse Transcriptional Loop-mediated Isothermal Amplification (RT-LAMP) technology coupled with a one-pot colorimetric visualization system to facilitate direct determination of results without additional steps. We also evaluated this multiplex RT-LAMP for clinical use using a total of 135 clinical and spiked samples (91 influenza viruses and 44 other human infectious viruses).

Results

We achieved rapid detection of seasonal influenza viruses (H1N1, H3N2, and Type B) and avian influenza viruses (H5N1, H5N6, H5N8 and H7N9) within an hour. The assay could detect influenza viruses with high sensitivity (i.e., from 100 to 0.1 viral genome copies), comparable to conventional RT-PCR-based approaches which would typically take several hours and require expensive equipment. This assay was capable of specifically detecting each influenza virus (Type B, H1N1, H3N2, H5N1, H5N6, H5N8 and H7N9) without cross-reactivity with other subtypes of AIVs or other human infectious viruses. Furthermore, 91 clinical and spiked samples confirmed by qRT-PCR were also detected by this multiplex RT-LAMP with 98.9% agreement. It was more sensitive than one-step RT-PCR approach (92.3%).

Conclusions

Results of this study suggest that our multiplex RT-LAMP assay may provide a rapid, sensitive, cost-effective, and reliable diagnostic method for identifying recent influenza viruses infecting humans, especially in locations without access to large platforms or sophisticated equipment.

Electronic supplementary material

The online version of this article (10.1186/s12879-019-4277-8) contains supplementary material, which is available to authorized users.

Humoral response to natural influenza infection in solid organ transplant recipients

The humoral immune response of transplant recipients to influenza vaccination has been studied in detail. In contrast, the hemagglutinin inhibiting (HI) antibody response evoked by natural influenza infection and its impact on viral kinetics is unknown. In this prospective, multicenter, cohort study of natural influenza infection in transplant recipients, we measured HI antibody titers at presentation and 4 weeks later. Serial nasopharyngeal viral loads were determined using a quantitative influenza A polymerase chain reaction (PCR). We analyzed 196 transplant recipients with influenza infection. In the cohort of organ transplant patients with influenza A (n = 116), seropositivity rates for strain-specific antibodies were 44.0% (95% confidence interval [CI] 31.5-53.2%) at diagnosis and 64.7% (95% CI 55.4-72.9%) 4 weeks postinfection. Seroconversion was observed in 32.8% (95% CI 24.7-41.9%) of the cases. Lung transplant recipients were more likely to seroconvert (P = .002) and vaccine recipients were less likely to seroconvert (P = .024). A subset of patients (n = 30) who were unresponsive to prior vaccination were also unresponsive to natural infection. There was no correlation between viral kinetics and antibody response. This study provides novel data on the seroresponse to influenza infection in transplant patients and its relationship to a number of parameters including a prior vaccination status, virologic measures, and clinical variables.

Investigating the Legacy of the 1918 Influenza Pandemic in Age-Related Seroepidemiology and Immune Responses to Subsequent Influenza A(H1N1) Viruses Through a Structural Equation Model

A(H1N1) strains of Influenzavirus were responsible for 2 pandemics in the last 100 years. Because infections experienced early in life may have a long-lasting influence on future immune response against other influenza strains, we drew on previously collected seroincidence data from Singapore (n = 2,554; June-October 2009) to investigate whether the 1918 pandemic influenza virus and its early descendants produced an age-related signature in immune responses against the A/California/7/2009(H1N1)pdm09 virus of 2009. Hemagglutination inhibition assays revealed a J-shaped relationship; the oldest birth cohort (born in 1911-1926) had the highest titers, followed by the youngest (born in 1987-1992). Differential response by vaccination history was also observed, with seasonal influenza vaccine being associated with higher titers mainly in the oldest birth cohort. On the assumption that antibody titers are a correlate of protection, structural equation modeling predicted that a titer-mediated effect by the vaccine could, on its own, account for a negative association with seroconversion equivalent to a risk reduction of 23% (relative risk = 0.77, 95% confidence interval: 0.60, 0.99) in the oldest birth cohort. A subset of 503 samples tested against the A/Brisbane/59/2007(H1N1) and A/Puerto Rico/8/1934(H1N1) strains also revealed different age-related antibody profiles. The effectiveness of seasonal influenza vaccines against future pandemic strains could thus be age-dependent and related to early-life exposures.

Epidemiology and Relative Severity of Influenza Subtypes in Singapore in the Post-Pandemic Period from 2009 to 2010

Background

After 2009, pandemic influenza A(H1N1) [A(H1N1)pdm09] cocirculated with A(H3N2) and B in Singapore.

Methods

A cohort of 760 participants contributed demographic data and up to 4 blood samples each from October 2009 to September 2010. We compared epidemiology of the 3 subtypes and investigated evidence for heterotypic immunity through multivariable logistic regression using a generalized estimating equation. To examine age-related differences in severity between subtypes, we used LOESS (locally weighted smoothing) plots of hospitalization to infection ratios and explored birth cohort effects referencing the pandemic years (1957; 1968).

Results

Having more household members aged 5–19 years and frequent public transport use increased risk of infection, while preexisting antibodies against the same subtype (odds ratio [OR], 0.61; P = .002) and previous influenza infection against heterotypic infections (OR, 0.32; P = .045) were protective. A(H1N1)pdm09 severity peaked in those born around 1957, while A(H3N2) severity was least in the youngest individuals and increased until it surpassed A(H1N1)pdm09 in those born in 1952 or earlier. Further analysis showed that severity of A(H1N1)pdm09 was less than that for A(H3N2) in those born in 1956 or earlier (P = .021) and vice versa for those born in 1968 or later (P < .001), with no difference in those born between 1957 and 1967 (P = .632).

Conclusions

Our findings suggest that childhood exposures had long-term impact on immune responses consistent with the theory of antigenic sin. This, plus observations on short-term cross-protection, have implications for vaccination and influenza epidemic and pandemic mitigation strategies.

Atypical antibody responses to influenza

Influenza viruses undergo rapid antigenic evolution and reassortment, resulting in annual epidemics and the occasional pandemics. Exposure to influenza virus hemagglutinin (HA) and neuraminidase (NA) antigen, either through vaccination or infection, induces an antibody response able to recognize only the homologous antigenic subtype. However, atypical antibody responses recognizing non-homologous influenza subtypes have been reported during infection and vaccination. Here, we review the incidence of these phenomena in published literature and discuss the potential mechanisms underlying them.

Low population serum microneutralization antibody titer against the predominating influenza A(H3N2) N121K virus during the severe influenza summer peak of Hong Kong in 2017

The 2017 Hong Kong influenza A(H3N2) summer season was unexpectedly severe. However, antigenic characterization of the 2017 circulating A(H3N2) viruses using ferret antisera did not show significant antigenic drift. We analyzed the hemagglutinin amino acid sequences of A(H3N2) virus circulating in Hong Kong in 2017, and found that viruses with hemagglutinin N121K substitution, which was rare before 2017, emerged rapidly and dominated in 2017 (52.4% of A[H3N2] virus in 2017 contains N121K substitution). Microneutralization assay using archived human sera collected from mid-2017 showed that the geometric mean microneutralization titer was 3.6-fold lower against a 2017 cell culture-grown circulating A(H3N2)-N121K virus (3391/2017 virus) than that against the cell culture-grown 2016–2017 A(H3N2) seasonal influenza vaccine-like vaccine virus (4801/2014 virus) (13.4 vs 41.8, P < 0.0001). Significantly fewer serum specimens had a microneutralization titer of 40 or above against 3391/2017 virus than that against 4801/2014 virus (26.4% vs 60.0%, P < 0.0001). Conversely, the geometric mean hemagglutination inhibition titer was slightly higher against 3391/2017 virus than that against the 4801/2014 virus (96.9 vs 55.4, P < 0.0001). Moreover, 59.1% of specimens had a significantly lower microneutralization antibody titer (≥4-fold) against 3391/2017 virus than that against 4801/2014 virus, but none for hemagglutination titer (P < 0.0001). Similar results of microneutralization and hemagglutination titers were observed for day 21-post-vaccination sera. Hence, the 2017 A(H3N2) summer peak in Hong Kong was associated with a low-microneutralization titer against the circulating virus. Our results support the use of microneutralization assay with human serum in assessing population susceptibility and antigenic changes of A(H3N2) virus. Novel and available immunization approach, such as topical imiquimod followed by intradermal vaccination, to broaden the neutralizing antibody response of influenza vaccine should be considered.

Individual and Population Trajectories of Influenza Antibody Titers Over Multiple Seasons in a Tropical Country

Seasonal influenza epidemics occur year-round in the tropics, complicating the planning of vaccination programs. We built an individual-level longitudinal model of baseline antibody levels, time of infection, and the subsequent rise and decay of antibodies postinfection using influenza A(H1N1)pdm09 data from 2 sources in Singapore: 1) a noncommunity cohort with real-time polymerase chain reaction–confirmed infections and at least 1 serological sample collected from each participant between May and October 2009 (n = 118) and 2) a community cohort with up to 6 serological samples collected between May 2009 and October 2010 (n = 760). The model was hierarchical, to account for interval censoring and interindividual variation. Model parameters were estimated via a reversible jump Markov chain Monte Carlo algorithm using custom-designed R (https://www.r-project.org/) and C++ (https://isocpp.org/) code. After infection, antibody levels peaked at 4–7 weeks, with a half-life of 26.5 weeks, followed by a slower decrease up to 1 year to approximately preinfection levels. After the third wave, the seropositivity rate and the population-level antibody titer dropped to the same level as they were at the end of the first pandemic wave. The results of this analysis are consistent with the hypothesis that the population-level effect of individuals’ waxing and waning antibodies influences influenza seasonality in the tropics.

Impact of Respiratory Virus Infections in Exacerbation of Acute and Chronic Rhinosinusitis

Rhinosinusitis (RS) is a symptomatic disease classification of many causes and is a major economic burden worldwide. It is widely accepted that RS is further classified into acute (ARS) and chronic (CRS) rhinosinusitis based on the duration of the symptoms, and that viral infection plays a large role in initiating or potentiating the disease. In this review, we examine the role of respiratory virus infection in the exacerbation of ARS and CRS. We explore the epidemiology of viral exacerbation of ARS and CRS and highlight key viruses that may cause exacerbation. We also review the current understanding of viral infections in the upper airway to further explain the putative underlying mechanisms of inflammatory events in ARS and CRS exacerbation. Advances in accurate diagnosis of the etiologic respiratory viruses of ARS and CRS symptoms which can lead to better disease management are also surveyed. In addition to the current treatments which provide symptomatic relief, we also explore the potential of harnessing existing antiviral strategies to prevent ARS and CRS exacerbation, especially with improved viral diagnostic tools to guide accurate prescription of antivirals against causative respiratory viruses.

A novel benzo-heterocyclic amine derivative N30 inhibits influenza virus replication by depression of Inosine-5'-Monophospate Dehydrogenase activity

BACKGROUD

Influenza virus is still a huge threat to the world-wide public health. Host inosine-5'- monophosphate dehydrogenase (IMPDH) involved in the synthesis of guanine nucleotides, is known to be a potential target to inhibit the replication of viruses. Herein, we evaluated antiviral activity of a benzo-heterocyclic amine derivative N30, which was designed to inhibit IMPDH.

RESULTS

The results demonstrated that N30 inhibited the replication of H1N1, H3N2, influenza B viruses, including oseltamivir and amantadine resistant strains in vitro. Mechanistically, neuraminidase inhibition assay and hemagglutination inhibition assay suggested that N30 did not directly target the two envelope glycoproteins required for viral adsorption or release. Instead, the compound could depress the activity of IMPDH type II. Based on these findings, we further confirmed that N30 provided a strong inhibition on the replication of respiratory syncytial virus, coronavirus, enterovirus 71 and a diverse strains of coxsackie B virus.

CONCLUSIONS

We identified the small molecule N30, as an inhibitor of IMPDH, might be a potential candidate to inhibit the replication of various viruses.

Quantifying homologous and heterologous antibody titre rises after influenza virus infection

Most influenza virus infections are associated with mild disease. One approach to estimate the occurrence of influenza virus infections in individuals is via repeated measurement of humoral antibody titres. We used baseline and convalescent antibody titres measured by haemagglutination inhibition (HI) and viral neutralization (VN) assays against influenza A(H1N1), A(H3N2) and B viruses to investigate the characteristics of antibody rises following virologically confirmed influenza virus infections in participants in a community-based study. Multivariate models were fitted in a Bayesian framework to characterize the distribution of changes in antibody titres following influenza A virus infections. In 122 participants with PCR-confirmed influenza A virus infection, homologous antibody titres rose by geometric means of 1·2- to 10·2-fold after infection with A(H1N1), A(H3N2) and A(H1N1)pdm09. Significant cross-reactions were observed between A(H1N1)pdm09 and seasonal A(H1N1). Antibody titre rises for some subtypes and assays varied by age, receipt of oseltamivir treatment, and recent receipt of influenza vaccination. In conclusion, we provided a quantitative description of the mean and variation in rises in influenza virus antibody titres following influenza virus infection. The multivariate patterns in boosting of antibody titres following influenza virus infection could be taken into account to improve estimates of cumulative incidence of infection in seroepidemiological studies.

Influenza, influenza-like symptoms and their association with cardiovascular risks: a systematic review and meta-analysis of observational studies

AIMS

To synthesise the evidence relating influenza and influenza-like symptoms to the risks of myocardial infarction (MI), heart failure (HF) and stroke.

METHODS

We conducted a systematic review and meta-analysis of the evidence relating influenza and influenza-like symptoms to the risks of MI, HF and stroke. We systematically searched all MEDLINE and EMBASE entries up to August 2014 for studies of influenza vs. the cardiovascular outcomes above. We conducted random effects meta-analysis using inverse variance method for pooled odds ratios (OR) and evaluated statistical heterogeneity using the I(2) statistic.

RESULTS

We identified 12 studies with a total of 84,003 participants. The pooled OR for risk of MI vs. influenza (serologically confirmed) was 1.27 (95% CI, confidence interval 0.54-2.95), I(2)  = 47%, which was significant for the only study that adjusted for confounders (OR 5.50, 95% CI 1.31-23.13). The pooled OR for risk of MI vs. influenza-like symptoms was 2.17 (95% CI 1.68-2.80), I(2)  = 0%, which was significant for both unadjusted (OR 2.23, 95% CI 1.65-3.01, five studies) and adjusted studies (OR 2.01, 95% CI 1.24-3.27, two studies). We found one study that evaluated stroke risk, one study in patients with HF, and one that evaluated mortality from MI - all of these studies suggested increased risks of events with influenza-like symptoms.

CONCLUSIONS

There is an association between influenza-like illness and cardiovascular events, but the relationship is less clear with serologically diagnosed influenza. We recommend renewed efforts to apply current clinical guidelines and maximise the uptake of annual influenza immunisation among patients with cardiovascular diseases, to decrease their risks of MI and stroke.

International Laboratory Comparison of Influenza Microneutralization Assays for A(H1N1)pdm09, A(H3N2), and A(H5N1) Influenza Viruses by CONSISE

The microneutralization assay is commonly used to detect antibodies to influenza virus, and multiple protocols are used worldwide. These protocols differ in the incubation time of the assay as well as in the order of specific steps, and even within protocols there are often further adjustments in individual laboratories. The impact these protocol variations have on influenza serology data is unclear. Thus, a laboratory comparison of the 2-day enzyme-linked immunosorbent assay (ELISA) and 3-day hemagglutination (HA) microneutralization (MN) protocols, using A(H1N1)pdm09, A(H3N2), and A(H5N1) viruses, was performed by the CONSISE Laboratory Working Group. Individual laboratories performed both assay protocols, on multiple occasions, using different serum panels. Thirteen laboratories from around the world participated. Within each laboratory, serum sample titers for the different assay protocols were compared between assays to determine the sensitivity of each assay and were compared between replicates to assess the reproducibility of each protocol for each laboratory. There was good correlation of the results obtained using the two assay protocols in most laboratories, indicating that these assays may be interchangeable for detecting antibodies to the influenza A viruses included in this study. Importantly, participating laboratories have aligned their methodologies to the CONSISE consensus 2-day ELISA and 3-day HA MN assay protocols to enable better correlation of these assays in the future.

Performance of case definitions for influenza surveillance

Influenza-like illness (ILI) case definitions, such as those from the European Centre for Disease Control and Prevention, World Health Organization (WHO) and United States Centers for Disease Control and Prevention, are commonly used for influenza surveillance. We assessed how various case definitions performed during the initial wave of influenza A(H1N1) pdm09 infections in Singapore on a cohort of 727 patients with two to three blood samples and whose symptoms were reviewed fortnightly from June to October 2009. Using seroconversion (≥ 4-fold rise) to A/California/7/2009 (H1N1), we identified 36 presumptive influenza A(H1N1)pdm09 episodes and 664 episodes unrelated to influenza A(H1N1)pdm09. Cough, fever and headache occurred more commonly in presumptive influenza A(H1N1)pdm09. Although the sensitivity was low (36%), the recently revised WHO ILI case definition gave a higher positive predictive value (42%) and positive likelihood ratio (13.3) than the other case definitions. Results including only episodes with primary care consultations were similar. Individuals who worked or had episodes with fever, cough or sore throat were more likely to consult a physician, while episodes with Saturday onset were less likely, with some consultations skipped or postponed. Our analysis supports the use of the revised WHO ILI case definition, which includes only cough in the presence of fever defined as body temperature ≥ 38 °C for influenza surveillance.

Statistical identifiability and sample size calculations for serial seroepidemiology

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We investigate whether disease dynamics can be inferred by repeated serum collections.

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Measuring antibody waning is critical for inference in serological time series.

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Collecting 200 samples every 2 months allows for inference of transmission parameters.

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Low-level seasonality is difficult to detect statistically.

Inference on disease dynamics is typically performed using case reporting time series of symptomatic disease. The inferred dynamics will vary depending on the reporting patterns and surveillance system for the disease in question, and the inference will miss mild or underreported epidemics. To eliminate the variation introduced by differing reporting patterns and to capture asymptomatic or subclinical infection, inferential methods can be applied to serological data sets instead of case reporting data. To reconstruct complete disease dynamics, one would need to collect a serological time series. In the statistical analysis presented here, we consider a particular kind of serological time series with repeated, periodic collections of population-representative serum. We refer to this study design as a serial seroepidemiology (SSE) design, and we base the analysis on our epidemiological knowledge of influenza. We consider a study duration of three to four years, during which a single antigenic type of influenza would be circulating, and we evaluate our ability to reconstruct disease dynamics based on serological data alone. We show that the processes of reinfection, antibody generation, and antibody waning confound each other and are not always statistically identifiable, especially when dynamics resemble a non-oscillating endemic equilibrium behavior. We introduce some constraints to partially resolve this confounding, and we show that transmission rates and basic reproduction numbers can be accurately estimated in SSE study designs. Seasonal forcing is more difficult to identify as serology-based studies only detect oscillations in antibody titers of recovered individuals, and these oscillations are typically weaker than those observed for infected individuals. To accurately estimate the magnitude and timing of seasonal forcing, serum samples should be collected every two months and 200 or more samples should be included in each collection; this sample size estimate is sensitive to the antibody waning rate and the assumed level of seasonal forcing.

Rate of decline of antibody titers to pandemic influenza A (H1N1-2009) by hemagglutination inhibition and virus microneutralization assays in a cohort of seroconverting adults in Singapore

Background

The rate of decline of antibody titers to influenza following infection can affect results of serological surveys, and may explain re-infection and recurrent epidemics by the same strain.

Methods

We followed up a cohort who seroconverted on hemagglutination inhibition (HI) antibody titers (≥4-fold increase) to pandemic influenza A(H1N1)pdm09 during a seroincidence study in 2009. Along with the pre-epidemic sample, and the sample from 2009 with the highest HI titer between August and October 2009 (A), two additional blood samples obtained in April 2010 and September 2010 (B and C) were assayed for antibodies to A(H1N1)pdm09 by both HI and virus microneutralization (MN) assays. We analyzed pair-wise mean-fold change in titers and the proportion with HI titers ≥ 40 and MN ≥ 160 (which correlated with a HI titer of 40 in our assays) at the 3 time-points following seroconversion.

Results

A total of 67 participants contributed 3 samples each. From the highest HI titer in 2009 to the last sample in 2010, 2 participants showed increase in titers (by HI and MN), while 63 (94%) and 49 (73%) had reduction in HI and MN titers, respectively. Titers by both assays decreased significantly; while 70.8% and 72.3% of subjects had titers of ≥ 40 and ≥ 160 by HI and MN in 2009, these percentages decreased to 13.9% and 36.9% by September 2010. In 6 participants aged 55 years and older, the decrease was significantly greater than in those aged below 55, so that none of the elderly had HI titers ≥ 40 nor MN titers ≥ 160 by the final sample. Due to this decline in titers, only 23 (35%) of the 65 participants who seroconverted on HI in sample A were found to seroconvert between the pre-epidemic sample and sample C, compared to 53 (90%) of the 59 who seroconverted on MN on Sample A.

Conclusions

We observed marked reduction in titers 1 year after seroconversion by HI, and to a lesser extent by MN. Our findings have implications for re-infections, recurrent epidemics, vaccination strategies, and for cohort studies measuring infection rates by seroconversion.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2334-14-414) contains supplementary material, which is available to authorized users.

Little evidence of subclinical avian influenza virus infections among rural villagers in Cambodia

In 2008, 800 adults living within rural Kampong Cham Province, Cambodia were enrolled in a prospective cohort study of zoonotic influenza transmission. After enrollment, participants were contacted weekly for 24 months to identify acute influenza-like illnesses (ILI). Follow-up sera were collected at 12 and 24 months. A transmission substudy was also conducted among the family contacts of cohort members reporting ILI who were influenza A positive. Samples were assessed using serological or molecular techniques looking for evidence of infection with human and avian influenza viruses. Over 24 months, 438 ILI investigations among 284 cohort members were conducted. One cohort member was hospitalized with a H5N1 highly pathogenic avian influenza (HPAI) virus infection and withdrew from the study. Ninety-seven ILI cases (22.1%) were identified as influenza A virus infections by real-time RT-PCR; none yielded evidence for AIV. During the 2 years of follow-up, 21 participants (3.0%) had detectable antibody titers (≥1∶10) against the studied AIVs: 1 against an avian-like A/Migratory duck/Hong Kong/MPS180/2003(H4N6), 3 against an avian-like A/Teal/Hong Kong/w312/97(H6N1), 9 (3 of which had detectible antibody titers at both 12- and 24-month follow-up) against an avian-like A/Hong Kong/1073/1999(H9N2), 6 (1 detected at both 12- and 24-month follow-up) against an avian-like A/Duck/Memphis/546/74(H11N9), and 2 against an avian-like A/Duck/Alberta/60/76(H12N5). With the exception of the one hospitalized cohort member with H5N1 infection, no other symptomatic avian influenza infections were detected among the cohort. Serological evidence for subclinical infections was sparse with only one subject showing a 4-fold rise in microneutralization titer over time against AvH12N5. In summary, despite conducting this closely monitored cohort study in a region enzootic for H5N1 HPAI, we were unable to detect subclinical avian influenza infections, suggesting either that these infections are rare or that our assays are insensitive at detecting them.

Inferring influenza infection attack rate from seroprevalence data

Seroprevalence survey is the most practical method for accurately estimating infection attack rate (IAR) in an epidemic such as influenza. These studies typically entail selecting an arbitrary titer threshold for seropositivity (e.g. microneutralization [MN] 1∶40) and assuming the probability of seropositivity given infection (infection-seropositivity probability, ISP) is 100% or similar to that among clinical cases. We hypothesize that such conventions are not necessarily robust because different thresholds may result in different IAR estimates and serologic responses of clinical cases may not be representative. To illustrate our hypothesis, we used an age-structured transmission model to fully characterize the transmission dynamics and seroprevalence rises of 2009 influenza pandemic A/H1N1 (pdmH1N1) during its first wave in Hong Kong. We estimated that while 99% of pdmH1N1 infections became MN1∶20 seropositive, only 72%, 62%, 58% and 34% of infections among age 3-12, 13-19, 20-29, 30-59 became MN1∶40 seropositive, which was much lower than the 90%-100% observed among clinical cases. The fitted model was consistent with prevailing consensus on pdmH1N1 transmission characteristics (e.g. initial reproductive number of 1.28 and mean generation time of 2.4 days which were within the consensus range), hence our ISP estimates were consistent with the transmission dynamics and temporal buildup of population-level immunity. IAR estimates in influenza seroprevalence studies are sensitive to seropositivity thresholds and ISP adjustments which in current practice are mostly chosen based on conventions instead of systematic criteria. Our results thus highlighted the need for reexamining conventional practice to develop standards for analyzing influenza serologic data (e.g. real-time assessment of bias in ISP adjustments by evaluating the consistency of IAR across multiple thresholds and with mixture models), especially in the context of pandemics when robustness and comparability of IAR estimates are most needed for informing situational awareness and risk assessment. The same principles are broadly applicable for seroprevalence studies of other infectious disease outbreaks.

Serological response of patients with influenza A (H1N1) pdm09-associated pneumonia: an observational study

Background

Little is known about the dynamics or magnitude of antibody response in patients with influenza A (H1N1) pdm09-associated pneumonia. We described and compared the antibody response to influenza A (H1N1) pdm09 in patients with and without pneumonia.

Methods

We collected serum samples and determined antibody titers by the hemagglutination inhibition (HI) and microneutralization (mNT) assays from patients with RT-PCR confirmed influenza A (H1N1) pdm09 virus at baseline, 1, 2 and 6 months after onset of illness.

Results

Fifty-nine patients were enrolled, 45 (76.3%) were between 15 and 60 years of age, 49 (83.1%) were hospitalized and 25 (42.4%) had complications with pneumonia. Ninety-four percent of patients had HI titers ≥ 1: 40 and 90% had mNT titers ≥ 1: 160 at 2 months after illness. Geometric mean titers (GMT) of HI and mNT increased significantly (p<0.001) between baseline and months 1 or 2, then declined significantly (p<0.001) at month 6 by the HI assay, but dropped to an insignificant level (p=0.24) by the mNT assay. The mNT-GMT was at least twice as high as corresponding HI antibodies over a 6 month period. The GMT of HI and mNT in those with pneumonia (1 mo) peaked earlier than that of those without pneumonia (2 mo). When adjusted by age and gender, those with pneumonia had a higher HI-GMT than those without pneumonia at 1 month (264 vs. 117, p=0.007), 2 months (212 vs. 159, p=0.013), and 6 months (160 vs. 82, p=0.018).

Conclusions

The patients recovered from influenza A (H1N1) pdm09-associated pneumonia, clearly developed an earlier and more robust antibody response until 6 months after onset of illness. The results in our study are useful to determine an appropriate donor and timing to obtain convalescent plasma for adjunctive treatment of seriously ill patients with pandemic H1N1 influenza.

Does homologous reinfection drive multiple-wave influenza outbreaks? Accounting for immunodynamics in epidemiological models

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We model the primary immune responses to influenza infection in humans.

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We examine the interplay between immunological and epidemiological dynamics.

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The model explains cases of homologous reinfection reported during past pandemics.

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Three epidemic profiles can arise depending on the degree of population mixing.

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A substantial proportion of infected host would remain unprotected after the 2009 influenza pandemic.

Epidemiological models of influenza transmission usually assume that recovered individuals instantly develop a fully protective immunity against the infecting strain. However, recent studies have highlighted host heterogeneity in the development of this immune response, characterized by delay and even absence of protection, that could lead to homologous reinfection (HR). Here, we investigate how these immunological mechanisms at the individual level shape the epidemiological dynamics at the population level. In particular, because HR was observed during the successive waves of past pandemics, we assess its role in driving multiple-wave influenza outbreaks. We develop a novel mechanistic model accounting for host heterogeneity in the immune response. Immunological parameters are inferred by fitting our dynamical model to a two-wave influenza epidemic that occurred on the remote island of Tristan da Cunha (TdC) in 1971, and during which HR occurred in 92 of 284 islanders. We then explore the dynamics predicted by our model for various population settings. We find that our model can explain HR over both short (e.g. week) and long (e.g. month) time-scales, as reported during past pandemics. In particular, our results reveal that the HR wave on TdC was a natural consequence of the exceptional contact configuration and high susceptibility of this small and isolated community. By contrast, in larger, less mixed and partially protected populations, HR alone cannot generate multiple-wave outbreaks. However, in the latter case, we find that a significant proportion of infected hosts would remain unprotected at the end of the pandemic season and should therefore benefit from vaccination. Crucially, we show that failing to account for these unprotected individuals can lead to large underestimation of the magnitude of the first post-pandemic season. These results are relevant in the context of the 2009 A/H1N1 influenza post-pandemic era.

Factors influencing infection by pandemic influenza A(H1N1)pdm09 over three epidemic waves in Singapore

Introduction

Previous influenza pandemics had second and on occasion third waves in many countries that were at times more severe than the initial pandemic waves.

Objective

This study aims to determine the seroepidemiology of successive waves of H1N1pdm09 infections in Singapore and the overall risks of infection.

Methods

We performed a cohort study amongst 838 adults, with blood samples provided upon recruitment and at 5 points from 2009 to 2011 and tested by haemagglutination inhibition (HI) with A/California/7/2009 (H1N1pdm09). Surveys on key demographic and clinical information were conducted at regular intervals, and associations between seroconversion and these variables were investigated.

Results

After the initial wave from June to September 2009, second and third waves occurred from November 2009 to February 2010 and April to June 2010, respectively. Seroconversion was 13·5% during the first wave and decreased to 6·2% and 6·8% in subsequent waves. Across the three waves, the elderly and those with higher starting HI titres were at lower risk of seroconversion, while those with larger households were at greater risk. Those with higher starting HI titres were also less likely to have an acute respiratory infection.

Conclusions

The second and third waves in Singapore had lower serological attack rates than the first wave. The elderly and those with higher HI titres had lower risk, while those in larger households had higher risk of seroconversion.

Seroepidemiologic effects of influenza A(H1N1)pdm09 in Australia, New Zealand, and Singapore

To estimate population attack rates of influenza A(H1N1)pdm2009 in the Southern Hemisphere during June-August 2009, we conducted several serologic studies. We pooled individual-level data from studies using hemagglutination inhibition assays performed in Australia, New Zealand, and Singapore. We determined seropositive proportions (titer ≥40) for each study region by age-group and sex in pre- and postpandemic phases, as defined by jurisdictional notification data. After exclusions, the pooled database consisted of, 4,414 prepandemic assays and 7,715 postpandemic assays. In the prepandemic phase, older age groups showed greater seropositive proportions, with age-standardized, community-based proportions ranging from 3.5% in Singapore to 11.9% in New Zealand. In the postpandemic phase, seropositive proportions ranged from 17.5% in Singapore to 30.8% in New Zealand, with highest proportions seen in school-aged children. Pregnancy and residential care were associated with lower postpandemic seropositivity, whereas Aboriginal and Torres Strait Islander Australians and Pacific Peoples of New Zealand had greater postpandemic seropositivity.

Tools to detect influenza virus

In 2009, pandemic influenza A (H1N1) virus (H1N1 09) started to spread quickly in many countries. It causes respiratory infection with signs and symptoms of common infectious agents. Thus, clinicians sometimes may miss the H1N1 patient. Clinical laboratory tests are important for the diagnosis of the H1N1 infection. There are several tests available, however, the rapid test and direct fluorescence antigen test are unable to rule out the influenza virus infection and viral culture test is time consuming. Therefore, nucleic acid amplification techniques based on reverse transcription polymerase chain reaction assays are regarded as a specific diagnosis to confirm the influenza virus infection. Although the nucleic acid-based techniques are highly sensitive and specific, the high mutation rate of the influenza RNA-dependent RNA polymerase could limit the utility of the techniques. In addition, their use depends on the availability, cost and throughput of the diagnostic techniques. To overcome these drawbacks, evaluation and development of the techniques should be continued. This review provides an overview of various techniques for specific diagnosis of influenza infection.

Influenza serological studies to inform public health action: best practices to optimise timing, quality and reporting

BACKGROUND

Serological studies can detect infection with a novel influenza virus in the absence of symptoms or positive virology, providing useful information on infection that goes beyond the estimates from epidemiological, clinical and virological data. During the 2009 A(H1N1) pandemic, an impressive number of detailed serological studies were performed, yet the majority of serological data were available only after the first wave of infection. This limited the ability to estimate the transmissibility and severity of this novel infection, and the variability in methodology and reporting limited the ability to compare and combine the serological data.

OBJECTIVES

  To identify best practices for conduct and standardisation of serological studies on outbreak and pandemic influenza to inform public policy.

METHODS/SETTING

An international meeting was held in February 2011 in Ottawa, Canada, to foster the consensus for greater standardisation of influenza serological studies.

RESULTS

Best practices for serological investigations of influenza epidemiology include the following: classification of studies as pre-pandemic, outbreak, pandemic or inter-pandemic with a clearly identified objective; use of international serum standards for laboratory assays; cohort and cross-sectional study designs with common standards for data collection; use of serum banks to improve sampling capacity; and potential for linkage of serological, clinical and epidemiological data. Advance planning for outbreak studies would enable a rapid and coordinated response; inclusion of serological studies in pandemic plans should be considered.

CONCLUSIONS

Optimising the quality, comparability and combinability of influenza serological studies will provide important data upon emergence of a novel or variant influenza virus to inform public health action.

Estimating age-specific cumulative incidence for the 2009 influenza pandemic: a meta-analysis of A(H1N1)pdm09 serological studies from 19 countries

Background

The global impact of the 2009 influenza A(H1N1) pandemic (H1N1pdm) is not well understood.

Objectives

We estimate overall and age‐specific prevalence of cross‐reactive antibodies to H1N1pdm virus and rates of H1N1pdm infection during the first year of the pandemic using data from published and unpublished H1N1pdm seroepidemiological studies.

Methods

Primary aggregate H1N1pdm serologic data from each study were stratified in standardized age groups and evaluated based on when sera were collected in relation to national or subnational peak H1N1pdm activity. Seropositivity was assessed using well‐described and standardized hemagglutination inhibition (HI titers ≥32 or ≥40) and microneutralization (MN ≥ 40) laboratory assays. The prevalence of cross‐reactive antibodies to the H1N1pdm virus was estimated for studies using sera collected prior to the start of the pandemic (between 2004 and April 2009); H1N1pdm cumulative incidence was estimated for studies in which collected both pre‐ and post‐pandemic sera; and H1N1pdm seropositivity was calculated from studies with post‐pandemic sera only (collected between December 2009–June 2010).

Results

Data from 27 published/unpublished studies from 19 countries/administrative regions – Australia, Canada, China, Finland, France, Germany, Hong Kong SAR, India, Iran, Italy, Japan, Netherlands, New Zealand, Norway, Reunion Island, Singapore, United Kingdom, United States, and Vietnam – were eligible for inclusion. The overall age‐standardized pre‐pandemic prevalence of cross‐reactive antibodies was 5% (95%CI 3–7%) and varied significantly by age with the highest rates among persons ≥65 years old (14% 95%CI 8–24%). Overall age‐standardized H1N1pdm cumulative incidence was 24% (95%CI 20–27%) and varied significantly by age with the highest in children 5–19 (47% 95%CI 39–55%) and 0–4 years old (36% 95%CI 30–43%).

Conclusions

Our results offer unique insight into the global impact of the H1N1 pandemic and highlight the need for standardization of seroepidemiological studies and for their inclusion in pre‐pandemic preparedness plans. Our results taken together with recent global pandemic respiratory‐associated mortality estimates suggest that the case fatality ratio of the pandemic virus was approximately 0·02%.

Influenza infection rates, measurement errors and the interpretation of paired serology

Serological studies are the gold standard method to estimate influenza infection attack rates (ARs) in human populations. In a common protocol, blood samples are collected before and after the epidemic in a cohort of individuals; and a rise in haemagglutination-inhibition (HI) antibody titers during the epidemic is considered as a marker of infection. Because of inherent measurement errors, a 2-fold rise is usually considered as insufficient evidence for infection and seroconversion is therefore typically defined as a 4-fold rise or more. Here, we revisit this widely accepted 70-year old criterion. We develop a Markov chain Monte Carlo data augmentation model to quantify measurement errors and reconstruct the distribution of latent true serological status in a Vietnamese 3-year serological cohort, in which replicate measurements were available. We estimate that the 1-sided probability of a 2-fold error is 9.3% (95% Credible Interval, CI: 3.3%, 17.6%) when antibody titer is below 10 but is 20.2% (95% CI: 15.9%, 24.0%) otherwise. After correction for measurement errors, we find that the proportion of individuals with 2-fold rises in antibody titers was too large to be explained by measurement errors alone. Estimates of ARs vary greatly depending on whether those individuals are included in the definition of the infected population. A simulation study shows that our method is unbiased. The 4-fold rise case definition is relevant when aiming at a specific diagnostic for individual cases, but the justification is less obvious when the objective is to estimate ARs. In particular, it may lead to large underestimates of ARs. Determining which biological phenomenon contributes most to 2-fold rises in antibody titers is essential to assess bias with the traditional case definition and offer improved estimates of influenza ARs.

Each year, seasonal influenza is responsible for about three to five million severe illnesses and about 250,000 to 500,000 deaths worldwide. In order to assess the burden of disease and guide control policies, it is important to quantify the proportion of people infected by an influenza virus each year. Since infection usually leaves a “signature” in the blood of infected individuals (namely a rise in antibodies), a standard protocol consists in collecting blood samples in a cohort of subjects and determining the proportion of those who experienced such rise. However, because of inherent measurement errors, only large rises are accounted for in the standard 4-fold rise case definition. Here, we revisit this 70 year old and widely accepted and applied criterion. We present innovative statistical techniques to better capture the impact of measurement errors and improve our interpretation of the data. Our analysis suggests that the number of people infected by an influenza virus each year might be substantially larger than previously thought, with important implications for our understanding of the transmission and evolution of influenza – and the nature of infection.

Teacher led school-based surveillance can allow accurate tracking of emerging infectious diseases - evidence from serial cross-sectional surveys of febrile respiratory illness during the H1N1 2009 influenza pandemic in Singapore

BACKGROUND

Schools are important foci of influenza transmission and potential targets for surveillance and interventions. We compared several school-based influenza monitoring systems with clinic-based influenza-like illness (ILI) surveillance, and assessed the variation in illness rates between and within schools.

METHODS

During the initial wave of pandemic H1N1 (pdmH1N1) infections from June to Sept 2009 in Singapore, we collected data on nation-wide laboratory confirmed cases (Sch-LCC) and daily temperature monitoring (Sch-DTM), and teacher-led febrile respiratory illness reporting in 6 sentinel schools (Sch-FRI). Comparisons were made against age-stratified clinic-based influenza-like illness (ILI) data from 23 primary care clinics (GP-ILI) and proportions of ILI testing positive for pdmH1N1 (Lab-ILI) by computing the fraction of cumulative incidence occurring by epidemiological week 30 (when GP-ILI incidence peaked); and cumulative incidence rates between school-based indicators and sero-epidemiological pdmH1N1 incidence (estimated from changes in prevalence of A/California/7/2009 H1N1 hemagglutination inhibition titers ≥ 40 between pre-epidemic and post-epidemic sera). Variation in Sch-FRI rates in the 6 schools was also investigated through a Bayesian hierarchical model.

RESULTS

By week 30, for primary and secondary school children respectively, 63% and 79% of incidence for Sch-LCC had occurred, compared with 50% and 52% for GP-ILI data, and 48% and 53% for Sch-FRI. There were 1,187 notified cases and 7,588 episodes in the Sch-LCC and Sch-DTM systems; given school enrollment of 485,723 children, this represented 0.24 cases and 1.6 episodes per 100 children respectively. Mean Sch-FRI rate was 28.8 per 100 children (95% CI: 27.7 to 29.9) in the 6 schools. We estimate from serology that 41.8% (95% CI: 30.2% to 55.9%) of primary and 43.2% (95% CI: 28.2% to 60.8%) of secondary school-aged children were infected. Sch-FRI rates were similar across the 6 schools (23 to 34 episodes per 100 children), but there was widespread variation by classrooms; in the hierarchical model, omitting age and school effects was inconsequential but neglecting classroom level effects led to highly significant reductions in goodness of fit.

CONCLUSIONS

Epidemic curves from Sch-FRI were comparable to GP-ILI data, and Sch-FRI detected substantially more infections than Sch-LCC and Sch-DTM. Variability in classroom attack rates suggests localized class-room transmission.

The first cases of 2009 pandemic influenza A (H1N1) virus infection in the United States: a serologic investigation demonstrating early transmission

BACKGROUND

The first two laboratory-confirmed cases of 2009 pandemic influenza A (H1N1) virus (H1N1pdm09) infection were detected in San Diego (SD) and Imperial County (IC) in southern California, April 2009.

OBJECTIVES

To describe H1N1pdm09 infections and transmission early in the 2009 H1N1 pandemic.

PATIENTS/METHODS

We identified index case-patients from SD and IC with polymerase chain reaction (PCR)-confirmed H1N1pdm09 infections and investigated close contacts for a subset of case-patients from April 17-May 6, 2009. Acute and convalescent serum was collected. Serologic evidence for H1N1pdm09 infection was determined by microneutralization and hemagglutination inhibition assays.

RESULTS

Among 75 close contacts of seven index case-patients, three reported illness onset prior to patient A or B, including two patient B contacts and a third with no links to patient A or B. Among the 69 close contacts with serum collected >14 days after the onset of index case symptoms, 23 (33%) were seropositive for H1N1pdm09, and 8 (35%) had no fever, cough, or sore throat. Among 15 household contacts, 8 (53%) were seropositive for H1N1pdm09. The proportion of contacts seropositive for H1N1pdm09 was highest in persons aged 5-24 years (50%) and lowest in persons aged ≥ 50 years (13%) (P = 0·07).

CONCLUSIONS

By the end of April 2009, before H1N1pdm09 was circulating widely in the community, a third of persons with close contact to confirmed H1N1pdm09 cases had H1N1pdm09 infection in SD and IC. Three unrelated clusters during March 21-30 suggest that transmission of H1N1pdm09 had begun earlier in southern California.

Cumulative incidence of pandemic influenza A (H1N1) 2009 by a community-based serological cohort study in Selenghe Province, Mongolia

BACKGROUND

Large community outbreaks of pandemic A (H1N1) 2009 occurred between October and December 2009 in Mongolia. A serological study was conducted among the general population by testing paired sera collected before and after the first wave of pandemic in Selenghe province, Mongolia. None of the study participants had been vaccinated for pandemic A (H1N1) 2009 before the second samples were collected.

OBJECTIVE

The objective of this study was to estimate cumulative incidence of pandemic A (H1N1) 2009 in different age-groups of Selenghe province residents.

METHODS

After informed consent was obtained from apparently healthy volunteers, the paired sera and background information were collected. Antibody titers were measured using hemagglutinin inhibition (HI) and microneutralization (MN) assays for A/California/07/2009pdm. A fourfold rise in antibody titers was regarded as the evidence of infection.

RESULTS

The overall cumulative incidences in the study group for all ages were 28.8% (76/264) by HI, 35.2% (93/264) by MN, and 25.0% (66/264) by both HI and MN. Cumulative incidences of infection varied among age-groups, with children aged 2-4 and 5-9 years having high cumulative incidence of infection. Overall cumulative incidences of infection in the whole population were estimated to be 23.0% (4946/21460) by HI, 30.2% (6473/21460) by MN, and 18.8% (4036/21460) by both HI and MN.

CONCLUSIONS

This study indicates that about one-fourth of the total population in Selenghe province was infected with pandemic A (H1N1) 2009 virus during the first wave of the pandemic.

Antibody response of healthy children to pandemic A/H1N1/2009 influenza virus

Background

Little is known about the proportion of pediatric pandemic A/H1N1/2009 influenza cases who showed seroconversion, the magnitude of this seroconversion, or the factors that can affect the antibody level evoked by the pandemic A/H1N1/2009 influenza. Aims of this study were to analyse antibody responses and the factors associated with high antibody titres in a cohort of children with naturally acquired A/H1N1/2009 influenza infection confirmed by reverse-transcriptase polymerase chain reaction (RT-PCR).

Results

Demographic, clinical and virologic data were collected from 69 otherwise healthy children with pandemic A/H1N1/2009 influenza (27 females, mean age ± SD: 5.01 ± 4.55 years). Their antibody levels against pandemic A/H1N1/2009 and seasonal A/H1N1 influenza viruses were evaluated by measuring hemagglutination-inhibiting antibodies using standard assays. Sixty-four patients (92.8%) with pandemic A/H1N1/2009 influenza had A/H1N1/2009 antibody levels of ≥40, whereas only 28/69 (40.6%) were seroprotected against seasonal A/H1N1 influenza virus. Those who were seroprotected against seasonal A/H1N1 virus were significantly older, significantly more often hospitalised, had a diagnosis of pneumonia significantly more frequently, and were significantly more often treated with oseltamivir than those who were not seroprotected (p < 0.05). The children with the most severe disease (assessed on the basis of a need for hospitalisation and a diagnosis of pneumonia) had the highest antibody response against pandemic A/H1N1/2009 influenza virus.

Conclusions

Otherwise healthy children seem to show seroprotective antibody titres after natural infection with pandemic A/H1N1/2009 influenza virus. The strength of the immune response seems to be related to the severity of the disease, but not to previous seasonal A/H1N1 influenza immunity.

Pandemic (H1N1) 2009 risk for frontline health care workers

To determine whether frontline health care workers (HCWs) are at greater risk for contracting pandemic (H1N1) 2009 than nonclinical staff, we conducted a study of 231 HCWs and 215 controls. Overall, 79 (17.7%) of 446 had a positive antibody titer by hemagglutination inhibition, with 46 (19.9%) of 231 HCWs and 33 (15.3%) of 215 controls positive (OR 1.37, 95% confidence interval 0.84–2.22). Of 87 participants who provided a second serum sample, 1 showed a 4-fold rise in antibody titer; of 45 patients who had a nose swab sample taken during a respiratory illness, 7 had positive results. Higher numbers of children in a participant’s family and working in an intensive care unit were risk factors for infection; increasing age, working at hospital 2, and wearing gloves were protective factors. This highly exposed group of frontline HCWs was no more likely to contract pandemic (H1N1) 2009 influenza infection than nonclinical staff, which suggests that personal protective measures were adequate in preventing transmission.

The age-specific cumulative incidence of infection with pandemic influenza H1N1 2009 was similar in various countries prior to vaccination

BACKGROUND

During the influenza pandemic of 2009 estimates of symptomatic and asymptomatic infection were needed to guide vaccination policies and inform other control measures. Serological studies are the most reliable way to measure influenza infection independent of symptoms. We reviewed all published serological studies that estimated the cumulative incidence of infection with pandemic influenza H1N1 2009 prior to the initiation of population-based vaccination against the pandemic strain.

METHODOLOGY AND PRINCIPAL FINDINGS

We searched for studies that estimated the cumulative incidence of pandemic influenza infection in the wider community. We excluded studies that did not include both pre- and post-pandemic serological sampling and studies that included response to vaccination. We identified 47 potentially eligible studies and included 12 of them in the review. Where there had been a significant first wave, the cumulative incidence of pandemic influenza infection was reported in the range 16%-28% in pre-school aged children, 34%-43% in school aged children and 12%-15% in young adults. Only 2%-3% of older adults were infected. The proportion of the entire population infected ranged from 11%-18%. We re-estimated the cumulative incidence to account for the small proportion of infections that may not have been detected by serology, and performed direct age-standardisation to the study population. For those countries where it could be calculated, this suggested a population cumulative incidence in the range 11%-21%.

CONCLUSIONS AND SIGNIFICANCE

Around the world, the cumulative incidence of infection (which is higher than the cumulative incidence of clinical disease) was below that anticipated prior to the pandemic. Serological studies need to be routine in order to be sufficiently timely to provide support for decisions about vaccination.

Sero-prevalence and incidence of A/H1N1 2009 influenza infection in Scotland in winter 2009-2010

Background

Sero-prevalence is a valuable indicator of prevalence and incidence of A/H1N1 2009 infection. However, raw sero-prevalence data must be corrected for background levels of cross-reactivity (i.e. imperfect test specificity) and the effects of immunisation programmes.

Methods and Findings

We obtained serum samples from a representative sample of 1563 adults resident in Scotland between late October 2009 and April 2010. Based on a microneutralisation assay, we estimate that 44% (95% confidence intervals (CIs): 40–47%) of the adult population of Scotland were sero-positive for A/H1N1 2009 influenza by 1 March 2010. Correcting for background cross-reactivity and for recorded vaccination rates by time and age group, we estimated that 34% (27–42%) were naturally infected with A/H1N1 2009 by 1 March 2010. The central estimate increases to >40% if we allow for imperfect test sensitivity. Over half of these infections are estimated to have occurred during the study period and the incidence of infection in late October 2009 was estimated at 4.3 new infections per 1000 people per day (1.2 to 7.2), falling close to zero by April 2010. The central estimate increases to over 5.0 per 1000 if we allow for imperfect test specificity. The rate of infection was higher for younger adults than older adults. Raw sero-prevalences were significantly higher in more deprived areas (likelihood ratio trend statistic = 4.92,1 df, P = 0.03) but there was no evidence of any difference in vaccination rates.

Conclusions

We estimate that almost half the adult population of Scotland were sero-positive for A/H1N1 2009 influenza by early 2010 and that the majority of these individuals (except in the oldest age classes) sero-converted as a result of natural infection with A/H1N1 2009. Public health planning should consider the possibility of higher rates of infection with A/H1N1 2009 influenza in more deprived areas.

Serological study of the 2009 pandemic due to influenza A H1N1 in the metropolitan French population

We looked for evidence of antibodies to the 2009 influenza A/H1N1 pandemic virus in panels of sera from individuals living in metropolitan France, obtained either before, during or after the epidemic, using standard haemagglutination inhibition and microneutralization tests. The difference between seroprevalence values measured in post- and pre-epidemic panels was used as an estimate of seroconversion rate in different age groups (23.4% (0-24 years, age-group 0); 16.5% (25-34); 7.9% (35-44); 7.2% (45-54); 1.6% (55-64); and 3.1% (>65)), confirming that the distribution of cases in different age groups was similar to that of the seasonal H1N1 virus. During the pre-pandemic period low-titre cross-reactive antibodies were present in a large proportion of the population (presumably acquired against seasonal H1N1) whereas cross-reactive antibodies were detected in individuals over the age of 65 years with significantly higher prevalence and serological titres (presumably acquired previously against Spanish flu-related H1N1 strains). Clinical data and analysis of post-pandemic seroprevalence showed that few of these latter patients were infected by the influenza virus during the epidemic. In contrast, the majority of both clinical cases and seroconversions were recorded in the 0-24 age group and a global inverse relationship between prevalence of antibodies to pH1N1 in the pre-pandemic period and rate of seroconversion was observed amongst age groups. Our results emphasize the complex relationships involved in antigenic reactivity to pandemic and seasonal H1N1 viral antigens; hence the difficulty in distinguishing between low-titre specific and cross-reactive antibodies, establishing precise seroprevalence numbers and fully understanding the relationship between previous immunity to seasonal viruses and protection against the novel variant.

Sensitivity and specificity of serologic assays for detection of human infection with 2009 pandemic H1N1 virus in U.S. populations

Swine origin 2009 H1N1 influenza virus has spread globally to cause the first influenza pandemic of the 21st century. Serological studies can improve our understanding of the extent of human infection and risk factors associated with the transmission of this pandemic virus. The "gold standard" for serodiagnosis of human influenza virus infection is the detection of seroconversion between acute- and convalescent-stage samples. However, the timing of seroepidemiological investigations often precludes the collection of truly acute-phase sera, requiring development of serological criteria for evaluating convalescent-phase sera that optimize detection of true positives and true negatives. To guide seroepidemiological investigations into the spread of the novel 2009 pandemic H1N1 virus, we characterized serum antibody responses to 2009 H1N1 virus in 87 individuals with confirmed viral infection and 227 nonexposed U.S. individuals using microneutralization (MN) and hemagglutination inhibition (HI) assays. Sensitivity and specificity were determined for each assay alone and in combination for detection of 2009 H1N1 virus-specific antibodies in convalescent-phase sera. Although the HI assay was more specific for detecting antibody to 2009 H1N1, the MN assay was more sensitive, particularly for detecting low-titer seroconversions. A combination of titers (MN ≥ 40 and HI ≥ 20) provided the highest sensitivity (90%) and specificity (96%) for individuals aged <60 years and 92% specificity for adults aged ≥ 60 years for detection of serologically confirmed 2009 H1N1 infections in U.S. populations during the first pandemic waves. These studies provide an approach to optimize timely serological investigations for future pandemics or outbreaks of novel influenza viruses among humans.

Serological responses following influenza A H1N1 2009 infection in adults

OBJECTIVE

The aim of this study was to determine the humoral immune response to influenza A H1N1 2009 and cross reactivity against seasonal H1N1 and H3N2 strains.

METHODS

Baseline and convalescent sera from 88 subjects with confirmed H1N1 2009 were screened for serological responses by HAI assay.

RESULTS

Protective antibody titres to H1N1 2009 were present in 87% post-infection, but varied with age, sex and pregnancy. Some cross reactivity with seasonal influenza strains was observed.

CONCLUSIONS

Females and pregnant subjects had an attenuated immune response to H1N1 2009 in comparison to the rest of the study population. Antibodies from the serum of H1N1 2009 infected subjects cross reacted with seasonal H1N1 and H3N2 influenza viruses.

Seroprevalence of pandemic (H1N1) 2009 in pregnant women in China: an observational study

Background

We investigated the seropositive rates and persistence of antibody against pandemic (H1N1) 2009 virus (pH1N1) in pregnant women and voluntary blood donors after the second wave of the pandemic in Nanjing, China.

Methodology/Principal Findings

Serum samples of unvaccinated pregnant women (n = 720) and voluntary blood donors (n = 320) were collected after the second wave of 2009 pandemic in Nanjing. All samples were tested against pH1N1 strain (A/California/7/2009) with hemagglutination inhibition assay. A significant decline in seropositive rates, from above 50% to about 20%, was observed in pregnant women and voluntary blood donors fifteen weeks after the second wave of the pandemic. A quarter of the samples were tested against a seasonal H1N1 strain (A/Brisbane/59/2007). The antibody titers against pH1N1 strain were found to correlate positively with those against seasonal H1N1 strain. The correlation was modest but statistically significant.

Conclusions and Significance

The high seropositive rates in both pregnant women and voluntary blood donors suggested that the pH1N1 virus had widely spread in these two populations. Immunity derived from natural infection seemed not to be persistent well.

Differences in antibody responses of individuals with natural infection and those vaccinated against pandemic H1N1 2009 influenza

The differential antibody response measured by the commonly used hemagglutination inhibition (HI) and microneutralization (MN) assays in patients with natural infection and vaccination has not been fully assessed. HI and conventional MN (CMN) assays were performed on sera from 651 patients with natural infection by pandemic H1N1 2009 influenza virus and on sera from 567 recipients of the corresponding vaccine. Surprisingly, the overall seroprotection rates determined by CMN and HI assays in vaccine recipients were only 44.8 and 35.1%, respectively. Antibody titers measured by the CMN assay was significantly higher than that obtained by HI assay in vaccine recipients aged ≥50 years, but these titers were not significantly different among younger vaccine recipients. In contrast, the HI titer was greater than the CMN titer for the age group from 16 to 29 years but was not significantly different in other age groups for natural infection. Lower antibody levels were found in both naturally infected patients and immunized recipients in the older than in the younger age groups, but naturally infected patients exhibited higher HI and CMN titers than did the corresponding vaccine recipients. In addition, we developed a rapid fluorescent focus microneutralization (FFMN) assay to test sera from naturally infected patients. The FFMN assay has a better correlation with CMN than with HI (ρ = 0.810 versus 0.684), which is expected of neutralizing antibody mainly targeted toward the inhibition of viral entry into cells. The higher antibody level elicited by natural infection than by vaccination may be related to differences between antigen presentation by the intramuscular route of vaccination and mucosal viral replication in mucosal cells of the respiratory tract.

Serological response to the 2009 pandemic influenza A (H1N1) virus for disease diagnosis and estimating the infection rate in Thai population

Background

Individuals infected with the 2009 pandemic virus A(H1N1) developed serological response which can be measured by hemagglutination-inhibition (HI) and microneutralization (microNT) assays.

Methodology/Principal Findings

MicroNT and HI assays for specific antibody to the 2009 pandemic virus were conducted in serum samples collected at the end of the first epidemic wave from various groups of Thai people: laboratory confirmed cases, blood donors and health care workers (HCW) in Bangkok and neighboring province, general population in the North and the South, as well as archival sera collected at pre- and post-vaccination from vaccinees who received influenza vaccine of the 2006 season. This study demonstrated that goose erythrocytes yielded comparable HI antibody titer as compared to turkey erythrocytes. In contrast to the standard protocol, our investigation found out the necessity to eliminate nonspecific inhibitor present in the test sera by receptor destroying enzyme (RDE) prior to performing microNT assay. The investigation in pre-pandemic serum samples showed that HI antibody was more specific to the 2009 pandemic virus than NT antibody. Based on data from pre-pandemic sera together with those from the laboratory confirmed cases, HI antibody titers ≥40 for adults and ≥20 for children could be used as the cut-off level to differentiate between the individuals with or without past infection by the 2009 pandemic virus.

Conclusions/Significance

Based on the cut-off criteria, the infection rates of 7 and 12.8% were estimated in blood donors and HCW, respectively after the first wave of the 2009 influenza pandemic. Among general population, the infection rate of 58.6% was found in children versus 3.1% in adults.