Mucosal correlates of protection after influenza viral challenge of vaccinated and unvaccinated healthy volunteers

The induction of systemic antibody titers against hemagglutinin has long been the main focus of influenza vaccination strategies, but mucosal immunity has also been shown to play a key role in the protection against respiratory viruses. By vaccinating and challenging healthy volunteers, we demonstrated that inactivated influenza vaccine (IIV) modestly reduced the rate of influenza while predominantly boosting serum antibody titers against hemagglutinin (HA) and HA stalk, a consequence of the low neuraminidase (NA) content of IIV and the intramuscular route of administration. The viral challenge induced nasal and serum responses against both HA and NA. Correlations between mucosal IgA and serum IgG against specific antigens were low, whether before or after challenge, suggesting a compartmentalization of immune responses. Even so, volunteers who developed viral shedding for multiple days had lower baseline titers across both systemic and mucosal compartments as compared to those with no shedding or a single day of shedding. Regression analysis showed that pre-challenge HA inhibition titers were the most consistent correlate of protection across clinical outcomes combining shedding and symptoms, with NA inhibition titers and HA IgG levels only predicting the duration of shedding. Despite the inclusion of data from multiple binding and functional antibody assays against HA and NA performed on both serum and nasal samples, multivariate models were unable to account for the variability in outcomes, emphasizing our imperfect understanding of immune correlates in influenza and the importance of refining models with assessments of innate and cellular immune responses.IMPORTANCEThe devastating potential of influenza has been well known for over 100 years. Despite the development of vaccines since the middle of the 20th century, influenza continues to be responsible for substantial global morbidity and mortality. To develop next-generation vaccines with enhanced effectiveness, we must synthesize our understanding of the complex immune mechanisms culminating in protection. Our study outlines the differences in immune responses to influenza vaccine and influenza infection, identifying potential gaps in vaccine-induced immunity, particularly at the level of the nasal mucosa. Furthermore, this research underscores the need to refine our imperfect models while recognizing potential pitfalls in past and future attempts to identify and measure correlates of protection.

Dynamics of SARS-CoV-2 Seroprevalence in a Large US population Over a Period of 12 Months

Due to a combination of asymptomatic or undiagnosed infections, the proportion of the United States population infected with SARS-CoV-2 was unclear from the beginning of the pandemic. We previously established a platform to screen for SARS-CoV-2 positivity across a representative proportion of the US population, from which we reported that almost 17 million Americans were estimated to have had undocumented infections in the Spring of 2020. Since then, vaccine rollout and prevalence of different SARS-CoV-2 variants have further altered seropositivity trends within the United States population. To explore the longitudinal impacts of the pandemic and vaccine responses on seropositivity, we re-enrolled participants from our baseline study in a 6- and 12- month follow-up study to develop a longitudinal antibody profile capable of representing seropositivity within the United States during a critical period just prior to and during the initiation of vaccine rollout. Initial measurements showed that, since July 2020, seropositivity elevated within this population from 4.8% at baseline to 36.2% and 89.3% at 6 and 12 months, respectively. We also evaluated nucleocapsid seropositivity and compared to spike seropositivity to identify trends in infection versus vaccination relative to baseline. These data serve as a window into a critical timeframe within the COVID-19 pandemic response and serve as a resource that could be used in subsequent respiratory illness outbreaks.

505. Mucosal and Systemic Humoral Immunity Differences between Sexes during Influenza Vaccination and Viral Challenge

Background

In previous influenza challenge studies we observed that the incidence of influenza symptoms was higher in women than in men, and that women experience more symptoms overall. Women also demonstrated lower neuraminidase (NA) inhibition (NAI) antibody titers than men after challenge and regression analysis suggested that NAI titers predict clinical outcomes. We sought to evaluate this using data from a more recent challenge study with more immunologically varied volunteers, while also assessing mucosal immunity.

Methods

We obtained mucosal and serum samples from healthy adults. Half underwent intramuscular quadrivalent influenza vaccination followed by influenza A (H1N1) intranasal challenge while others just challenge alone. Antibody titers and clinical outcomes between sexes were compared.

Results

No significant differences in shedding or symptom outcomes were observed between sexes. Similarly, no differences in systemic titers against NA or hemagglutinin (HA) were seen. Serum total IgG and IgA were also similar between sexes. Prior to vaccination, mucosal IgA titers were significantly lower against HA stalk and NA in women, but these differences disappeared after vaccination. In the unvaccinated group, lower nasal IgA titers were also seen in women pre-challenge. At 7 days post-challenge, differences in mucosal titers between sexes disappeared. However, at 56 days post-challenge, mucosal IgA titers against HA, HA stalk, and NA were all significantly lower in women.

Conclusion

In contrast to our previous findings, no differences in clinical outcomes or systemic NAI titers were observed between men and women, possibly due to differences in underlying immunity between study populations or the lower sample size in this study. However, mucosal differences were noted with women having lower mucosal titers than men against HA stalk and NA at baseline. While influenza vaccination and challenge eliminated differences in the short term, they reappeared 8 weeks after challenge, with all nasal IgA antibody titers (HA, HA stalk, and NA) lower in women. Further studies are necessary to understand the differences in influenza disease and immunity between sexes.

Disclosures

All Authors: No reported disclosures.

Undiagnosed SARS-CoV-2 seropositivity during the first 6 months of the COVID-19 pandemic in the United States

Asymptomatic SARS-CoV-2 infection and delayed implementation of diagnostics have led to poorly defined viral prevalence rates in the United States and elsewhere. To address this, we analyzed seropositivity in 9089 adults in the United States who had not been diagnosed previously with COVID-19. Individuals with characteristics that reflected the U.S. population (n = 27,716) were selected by quota sampling from 462,949 volunteers. Enrolled participants (n = 11,382) provided medical, geographic, demographic, and socioeconomic information and dried blood samples. Survey questions coincident with the Behavioral Risk Factor Surveillance System survey, a large probability-based national survey, were used to adjust for selection bias. Most blood samples (88.7%) were collected between 10 May and 31 July 2020 and were processed using ELISA to measure seropositivity (IgG and IgM antibodies against SARS-CoV-2 spike protein and the spike protein receptor binding domain). The overall weighted undiagnosed seropositivity estimate was 4.6% (95% CI, 2.6 to 6.5%), with race, age, sex, ethnicity, and urban/rural subgroup estimates ranging from 1.1% to 14.2%. The highest seropositivity estimates were in African American participants; younger, female, and Hispanic participants; and residents of urban centers. These data indicate that there were 4.8 undiagnosed SARS-CoV-2 infections for every diagnosed case of COVID-19, and an estimated 16.8 million infections were undiagnosed by mid-July 2020 in the United States.

Mapping a Pandemic: SARS-CoV-2 Seropositivity in the United States

Asymptomatic SARS-CoV-2 infection and delayed implementation of diagnostics have led to poorly defined viral prevalence rates. To address this, we analyzed seropositivity in US adults who have not previously been diagnosed with COVID-19. Individuals with characteristics that reflect the US population (n = 11,382) and who had not previously been diagnosed with COVID-19 were selected by quota sampling from 241,424 volunteers (ClinicalTrials.gov NCT04334954). Enrolled participants provided medical, geographic, demographic, and socioeconomic information and 9,028 blood samples. The majority (88.7%) of samples were collected between May 10th and July 31st, 2020. Samples were analyzed via ELISA for anti-Spike and anti-RBD antibodies. Estimation of seroprevalence was performed by using a weighted analysis to reflect the US population. We detected an undiagnosed seropositivity rate of 4.6% (95% CI: 2.6 - 6.5%). There was distinct regional variability, with heightened seropositivity in locations of early outbreaks. Subgroup analysis demonstrated that the highest estimated undiagnosed seropositivity within groups was detected in younger participants (ages 18-45, 5.9%), females (5.5%), Black/African American (14.2%), Hispanic (6.1%), and Urban residents (5.3%), and lower undiagnosed seropositivity in those with chronic diseases. During the first wave of infection over the spring/summer of 2020 an estimate of 4.6% of adults had a prior undiagnosed SARS-CoV-2 infection. These data indicate that there were 4.8 (95% CI: 2.8-6.8) undiagnosed cases for every diagnosed case of COVID-19 during this same time period in the United States, and an estimated 16.8 million undiagnosed cases by mid-July 2020.

Influenza A Reinfection in Sequential Human Challenge: Implications for Protective Immunity and "Universal" Vaccine Development

BACKGROUND

Identification of correlates of protection against human influenza A virus infection is important in development of broadly protective ("universal") influenza vaccines. Certain assumptions underlie current vaccine developmental strategies, including that infection with a particular influenza A virus should offer long-term or lifelong protection against that strain, preventing reinfection. In this study we report observations made when 7 volunteers participated in sequential influenza challenge studies where they were challenged intranasally using the identical influenza A(H1N1)pdm09 virus approximately 1 year apart. We evaluate and describe the outcomes of these 7 rechallenge participants and discuss what these results may suggest about correlates of protection and development of more broadly protective influenza vaccines.

METHODS

Seven participants were enrolled in 2 viral challenge studies at 7.5- to 18.5-month intervals. Both challenge studies used the identical lot of influenza A (H1N1)pdm09 virus administered intranasally. We evaluated pre- and postchallenge hemagglutination inhibition, neuraminidase inhibition, and stalk antibody titers; peripheral blood leukocyte host gene expression response profiles; daily viral detection via nasal wash; and clinical signs and symptoms.

RESULTS

At least 3 of 7 participants demonstrated confirmed laboratory evidence of sequential infection, with 5 of 7 demonstrating clinical evidence.

CONCLUSIONS

The data presented in this report demonstrate that sequential infection with the identical influenza A virus can occur and suggest it may not be rare. These data raise questions about immune memory responses in an acute superficial respiratory mucosal infection and their implications in development of broadly protective influenza vaccines. Further investigation of these observations is warranted.

CLINICAL TRIALS REGISTRATION

NCT01646138; NCT01971255.

1522. Safety and Efficacy of CR6261 in an Influenza A H1N1 Healthy Human Challenge Model

Background

Influenza virus infections cause significant morbidity and mortality during yearly seasonal epidemics and during sporadic pandemics. It is imperative to identify new targets for vaccines and therapeutics. One such target is the relatively conserved stalk region of the influenza A hemagglutinin (HA) surface protein.

Methods

We conducted a randomized, double-blind, Phase II placebo-controlled trial of a monoclonal antibody that targets the HA stalk (CR6261) in a H1N1pdm09 healthy volunteer human challenge model. CR6261 was infused 24 hours after challenge with H1N1pdm09 and the primary efficacy outcome was area under the curve (AUC) of viral shedding.

Results

Between March 2015-May 2018, 104 healthy volunteers were enrolled and randomized with 91 undergoing influenza challenge, of which 49 participants (54%) received treatment with CR6261 and 42 participants (46%) received placebo. A mean of 1x10⁶ ng/mL of serum CR6261 was detected by 24 hours after infusion. Nasal CR6261 levels reached a peak mean of 5.97x10² ng/ml 2 days after infusion. There was no statistically significant difference in the primary outcome measure between the CR6261 group and placebo (median AUC 48.56 and 25.53 respectively, P=0.31). The severity of illness was compared between the two groups, and no significant difference was observed in number of symptoms, duration of symptoms, or FLU-PRO scores.

Conclusion

CR6261 had no statistically significant effect on AUC of viral shedding, and no clinically significant effect on overall influenza disease. Preexisting anti-neuraminidase (NA) antibody titers were most predictive of reduced influenza disease. Nasal CR6261 levels were much lower compared to serum, which may be a factor in the limited effect of CR6261 on this upper respiratory infection. These results suggest that a monoclonal anti-stalk approach to prevent or treat influenza infection may have limited efficacy. Future approaches should consider including and evaluating anti-stalk antibodies as part of a multi-faceted strategy rather than as a standalone therapeutic or vaccine strategy.

Funding

This study was funded in part by the intramural program of NIAID, NIH, by the NCI Contract No. 75N910D00024, Task Order No. 75N91019F00130, and through a CRADA with Janssen Infectious Diseases and Vaccines.

Disclosures

Amy Lwin, RN, BSN, Janssen Pharmaceutical Company of J&J (Employee) Jerald Sadoff, MD, Janssen Pharmaceutical Company of J&J (Employee)

Safety and Efficacy of CR6261 in an Influenza A H1N1 Healthy Human Challenge Model

BACKGROUND

It is imperative to identify new targets for improved vaccines and therapeutics against influenza and one such target is the relatively conserved stalk region of the influenza A hemagglutinin (HA) surface protein.

METHODS

We conducted a randomized, double-blind, Phase II placebo-controlled trial of a monoclonal antibody that targets the HA stalk (CR6261) in a H1N1pdm09 healthy volunteer human challenge model. A single 50mg/kg dose of CR6261 was infused 24 hours after challenge and the primary efficacy outcome was area under the curve of viral RNA detection over time.

RESULTS

Ninety-one healthy volunteers were randomized and underwent influenza challenge; 49 received CR6261 and 42 placebo. CR6261 had no statistically significant effect on AUC (AUC 48.56 log (copies/mL) x days, IQR 202 vs. AUC 25.53 log (copies/mL) x days, IQR 155), P=0.315), and no clinically significant effect on influenza disease measures including number of symptoms, duration of symptoms, or FLU-PRO scores. Preexisting anti-NA antibody titers were most predictive of reduced influenza disease. CR6261 reached a mean peak serum concentration of 1x10 6 ng/ml 15 minutes after infusion, and a mean peak of 5.97x10 2 ng/ml in the nasal mucosa 2-3 days after infusion.

CONCLUSIONS

The results of this study suggest that a monoclonal anti-stalk approach to prevent or treat influenza infection may be limited in efficacy. Future approaches should consider including and evaluating anti-stalk antibodies as part of a multi-faceted strategy rather than as a standalone therapeutic.

Pre-existing immunity to influenza virus hemagglutinin stalk might drive selection for antibody-escape mutant viruses in a human challenge model

The conserved region of influenza hemagglutinin (HA) stalk (or stem) has gained attention as a potent target for universal influenza vaccines^1-5. Although the HA stalk region is relatively well conserved, the evolutionarily dynamic nature of influenza viruses⁶ raises concerns about the possible emergence of viruses carrying stalk escape mutation(s) under sufficient immune pressure. Here we show that immune pressure on the HA stalk can lead to expansion of escape mutant viruses in study participants challenged with a 2009 H1N1 pandemic influenza virus inoculum containing an A388V polymorphism in the HA stalk (45% wild type and 55% mutant). High level of stalk antibody titers was associated with the selection of the mutant virus both in humans and in vitro. Although the mutant virus showed slightly decreased replication in mice, it was not observed in cell culture, ferrets or human challenge participants. The A388V mutation conferred resistance to some of the potent HA stalk broadly neutralizing monoclonal antibodies (bNAbs). Co-culture of wild-type and mutant viruses in the presence of either a bNAb or human serum resulted in rapid expansion of the mutant. These data shed light on a potential obstacle for the success of HA-stalk-targeting universal influenza vaccines-viral escape from vaccine-induced stalk immunity.

Safety and immunogenicity of a mosquito saliva peptide-based vaccine: a randomised, placebo-controlled, double-blind, phase 1 trial

BACKGROUND

In animal models, immunity to mosquito salivary proteins protects animals against mosquito-borne disease. These findings provide a rationale to vaccinate against mosquito saliva instead of the pathogen itself. To our knowledge, no vector salivary protein-based vaccine has been tested for safety and immunogenicity in humans. We aimed to assess the safety and immunogenicity of Anopheles gambiae saliva vaccine (AGS-v), a peptide-based vaccine derived from four A gambiae salivary proteins, in humans.

METHODS

In this randomised, placebo-controlled, double-blind, phase 1 trial, participants were enrolled at the National Institutes of Health Clinical Center in Bethesda, MD, USA. Participants were eligible if they were healthy adults, aged 18-50 years with no history of severe allergic reactions to mosquito bites. Participants were randomly assigned (1:1:1), using block randomisation and a computer-generated randomisation sequence, to treatment with either 200 nmol of AGS-v vaccine alone, 200 nmol of AGS-v with adjuvant (Montanide ISA 51), or sterile water as placebo. Participants and clinicians were masked to treatment assignment. Participants were given a subcutaneous injection of their allocated treatment at day 0 and day 21, followed by exposure to feeding by an uninfected Aedes aegypti mosquito at day 42 to assess subsequent risk to mosquito bites in a controlled setting. The primary endpoints were safety and immunogenicity at day 42 after the first immunisation. Participants who were given at least one dose of assigned treatment were assessed for the primary endpoints and analysis was by intention to treat. The trial was registered with ClinicalTrials.gov, NCT03055000, and is closed for accrual.

FINDINGS

Between Feb 15 and Sept 10, 2017, we enrolled and randomly assigned 49 healthy adult participants to the adjuvanted vaccine (n=17), vaccine alone (n=16), or placebo group (n=16). Five participants did not complete the two-injection regimen with mosquito feeding at day 42, but were included in the safety analyses. No systemic safety concerns were identified; however, one participant in the adjuvanted vaccine group developed a grade 3 erythematous rash at the injection site. Pain, swelling, erythema, and itching were the most commonly reported local symptoms and were significantly increased in the adjuvanted vaccine group compared with both other treatment groups (nine [53%] of 17 participants in the adjuvanted vaccine group, two [13%] of 16 in the vaccine only group, and one [6%] of 16 in the placebo group; p=0·004). By day 42, participants who were given the adjuvanted vaccine had a significant increase in vaccine-specific total IgG antibodies compared with at baseline than did participants who were give vaccine only (absolute difference of log₁₀-fold change of 0·64 [95% CI 0·39 to 0·89]; p=0·0002) and who were given placebo (0·62 [0·34 to 0·91]; p=0·0001). We saw a significant increase in IFN-γ production by peripheral blood mononuclear cells at day 42 in the adjuvanted vaccine group compared with in the placebo group (absolute difference of log₁₀ ratio of vaccine peptide-stimulated vs negative control 0·17 [95% CI 0·061 to 0·27]; p=0·009) but we saw no difference between the IFN-γ production in the vaccine only group compared with the placebo group (0·022 [-0·072 to 0·116]; p=0·63).

INTERPRETATION

AGS-v was well tolerated, and, when adjuvanted, immunogenic. These findings suggest that vector-targeted vaccine administration in humans is safe and could be a viable option for the increasing burden of vector-borne disease.

FUNDING

Office of the Director and the Division of Intramural Research at the National Institute of Allergy and Infectious Diseases, and National Institutes of Health.

2750. Sequential Influenza A H1N1 and Influenza A H3N2 Challenge Infections in Healthy Volunteers

Background

Seasonal influenza causes significant annual morbidity and mortality. The effects of yearly exposures on immunity are not clear and recent observations have demonstrated that long lasting protection against a matched strain may not naturally occur. The 2018–2019 influenza season consisted of an initial peak of H1N1 infections followed by a wave of H3N2 infections. These consecutive waves raise questions about how influenza immunity is affected by sequential exposure to different influenza strains. Challenge studies provide a unique opportunity to study this phenomenon. Here we describe a subset of participants who were sequentially infected in two separate challenge studies with wild-type H1N1 and H3N2 viruses.

Methods

Healthy volunteers completed two sequential influenza challenge studies at the NIH Clinical Center. Participants were inoculated with reverse genetics, cell-based, GMP wild-type influenza viruses, A(H1N1)pdm09 and A(H3N2) strains. Participants remained isolated in the hospital for a minimum of 9 days and were monitored daily for viral shedding and clinical symptoms. After discharge, participants were followed for 2 months.

Results

Between 2014 and 2017, 14 healthy volunteers were exposed to Influenza A(H1N1) and Influenza A(H3N2). Time between infections ranged from 2 months to 2 years. Thirteen (93%) participants developed confirmed influenza infection after H1N1 challenge and 9 (64%) after H3N2 challenge. Eight (57%) participants developed confirmed infections after both exposures. Variable degrees of symptoms, shedding, and disease severity were observed. Systemic antibody responses to the HA and NA of both H1N1 and H3N2 varied over time during these sequential infections.

Conclusion

More than half of all participants who completed 2 sequential H1N1 and H3N2 challenge studies demonstrated confirmed infection to both viruses. These sequential infections had varying effects on the disease experienced and the immunity that developed after infection. These observations are important in understanding the impact of sequential exposures on influenza immunity.

Disclosures

All authors: No reported disclosures.

723. Validation of a Wild-Type Influenza A/Texas-Like H3N2 Human Challenge Model with Comparison to the Validated A(H1N1)pdm09 Model

Background

Healthy volunteer challenge studies provide an opportunity to better understand influenza pathogenesis and correlates of protection. The development of vaccines and therapeutics has relied on these studies as will future universal vaccine candidates. The first fully validated wild-type human infection model with A(H1N1)pdm09 was developed at the NIH Clinical Center (CC) in 2012 and this study represents the first validation of a wild-type seasonal H3N2 human infection model. The objective of this study was to characterize a wild-type Influenza A/Texas-like H3N2 challenge virus in healthy volunteers.

Methods

Healthy volunteers were isolated at the NIH CC for a minimum of 9 days. Subjects received a single dose of a reverse genetics, cell-based, GMP, wild-type A H3N2 virus intranasally. Dose escalation was performed from 104 to 107 TCID50. Viral shedding and clinical disease were evaluated daily, including clinician assessments and a validated patient-reported outcome tool, FLU-PRO©.

Results

A total of 37 subjects were challenged. Sixteen (43%) subjects had viral shedding and 27 (73%) developed influenza symptoms, with 12 subjects (32%) experiencing mild-to-moderate influenza disease (MMID) defined as symptoms and shedding. Only subjects receiving the 106 and 107 TCID50 doses experienced MMID at 44% and 40%, respectively. Nose and throat symptoms were most common and peaked by Days 2–3 post-challenge. Although serum antibody responses were observed, many of these responses were limited in a significant number of subjects.

Conclusion

The A/Texas-like H3N2 Influenza challenge virus safely induced MMID in healthy volunteers, but was less effective than the A(H1N1)pdm09 challenge virus. This lower MMID rate of 40% was observed at the 107 TCID50 dose and was driven by less detection of shedding as the incidence of symptoms was similar to A(H1N1)pdm09. The limited serum antibody responses observed demonstrate that preexisting immunity in healthy volunteers against the seasonal H3N2 lineage may limit shedding compared with the more recently emerged seasonal A(H1N1)pdm09 lineage. The successful characterization of this H3N2 model makes future studies using this model to explore viral pathogenesis or evaluate vaccines possible.

This research was supported by the Intramural Research Program of the NIH, NIAID.

Disclosures

All authors: No reported disclosures.

Validation of the wild-type influenza A human challenge model H1N1pdMIST: an A(H1N1)pdm09 dose-finding investigational new drug study

BACKGROUND

Healthy volunteer wild-type influenza challenge models offer a unique opportunity to evaluate multiple aspects of this important virus. Such studies have not been performed in the United States in more than a decade, limiting our capability to investigate this virus and develop countermeasures. We have completed the first ever wild-type influenza A challenge study under an Investigational New Drug application (IND). This dose-finding study will lead to further development of this model both for A(H1N1)pdm09 and other strains of influenza.

METHODS

Volunteers were admitted to an isolation unit at the National Institutes of Health Clinical Center for a minimum of 9 days. A reverse genetics, cell-based, Good Manufacturing Practice (GMP)-produced, wild-type A(H1N1)pdm09 virus was administered intranasally. Escalating doses were given until a dose was reached that produced disease in a minimum of 60% of volunteers.

RESULTS

An optimal dose of 10(7) tissue culture infectious dose 50 was reached that caused mild to moderate influenza disease in 69% of individuals with mean viral shedding for 4-5 days and significant rises in convalescent influenza antibody titers. Viral shedding preceded symptoms by 12-24 hours and terminated 2-3 days prior to symptom resolution, indicating that individuals may be infectious before symptom development. As expected, nasal congestion and rhinorrhea were most common, but interestingly, fever was observed in only 10% of individuals.

CONCLUSIONS

This study represents the first healthy volunteer influenza challenge model using a GMP-produced wild-type virus under an IND. This unique clinical research program will facilitate future studies of influenza pathogenesis, animal model validation, and the rapid, efficient, and cost-effective evaluation of efficacy of novel vaccines and therapeutics. Clinical Trials Registration.NCT01646138.

The natural history of influenza infection in the severely immunocompromised vs nonimmunocompromised hosts

Introduction. Medical advances have led to an increase in the world's population of immunosuppressed individuals. The most severely immunocompromised patients are those who have been diagnosed with a hematologic malignancy, solid organ tumor, or who have other conditions that require immunosuppressive therapies and/or solid organ or stem cell transplants.

Materials and methods. Medically attended patients with a positive clinical diagnosis of influenza were recruited prospectively and clinically evaluated. Nasal washes and serum were collected. Evaluation of viral shedding, nasal and serum cytokines, clinical illness, and clinical outcomes were performed to compare severely immunocompromised individuals to nonimmunocompromised individuals with influenza infection.

Results. Immunocompromised patients with influenza had more severe disease/complications, longer viral shedding, and more antiviral resistance while demonstrating less clinical symptoms and signs on clinical assessment.

Conclusions. Immunocompromised patients are at risk for more severe or complicated influenza induced disease, which may be difficult to prevent with existing vaccines and antiviral treatments. Specific issues to consider when managing a severely immunocompromised host include the development of asymptomatic shedding, multi-drug resistance during prolonged antiviral therapy, and the potential high risk of pulmonary involvement.

Clinical trials registration, ClinicalTrials.gov identifier NCT00533182.