Gradual changes in the age distribution of excess deaths in the years following the 1918 influenza pandemic in Copenhagen: using epidemiological evidence to detect antigenic drift

An extended period of elevated influenza mortality risk follows the main waves of influenza pandemics

Understanding the extent and evolution of pandemic-induced mortality risk is critical given its wide-ranging impacts on population health and socioeconomic outcomes. We examine empirically the persistence and scale of influenza mortality risk following the main waves of influenza pandemics, a quantitative analysis of which is required to understand the true scale of pandemic-induced risk. We provide evidence from municipal public health records that multiple recurrent outbreaks followed the main waves of the 1918-19 pandemic in eight large cities in the UK, a pattern we confirm using data for the same period in the US and data for multiple influenza pandemics during the period 1838-2000 in England and Wales. To estimate the persistence and scale of latent post-pandemic influenza mortality risk, we model the stochastic process of mortality rates as a sequence of bounded Pareto distributions whose tail indexes evolves over time. Consistently across pandemics and locations, we find that influenza mortality risk remains elevated for around two decades after the main pandemic waves before more rapid convergence to background influenza mortality, amplifying the impact of pandemics. Despite the commonality in duration, there is heterogeneity in the persistence and scale of risk across the cities, suggesting effects of both immunity and socioeconomic conditions.

Did the 1918 influenza pandemic cause a 1920 baby boom? Demographic evidence from neutral Europe

In 1919-20, the European countries that were neutral in the First World War saw a small baby bust followed by a small baby boom. The sparse literature on this topic attributes the 1919 bust to individuals postponing conceptions during the peak of the 1918-20 influenza pandemic and the 1920 boom to recuperation of those conceptions. Using data from six large neutral countries of Europe, we present novel evidence contradicting that narrative. In fact, the subnational populations and maternal birth cohorts whose fertility was initially hit hardest by the pandemic were still experiencing below-average fertility in 1920. Demographic evidence, economic evidence, and a review of post-pandemic fertility trends outside Europe suggest that the 1920 baby boom in neutral Europe was caused by the end of the First World War, not by the end of the pandemic.

Consequences of delayed care during the COVID-19 pandemic: Emerging research and new lines of inquiry for human biologists and anthropologists

OBJECTIVE

Research regarding ongoing epidemic or pandemic events is often proximate, focusing on the immediate need to understand the epidemiology of the outbreak and the populations at highest risk for negative outcomes. There are other characteristics of pandemics that can only be uncovered after time has passed, and some long-lasting health consequences may not be directly linked to infection with or disease from the pandemic pathogen itself.

METHODS

We discuss the emerging literature on observations delayed care during the COVID-19 pandemic and the potential population health consequences of this phenomenon in postpandemic years, especially for conditions such as cardiovascular disease, cancer, and reproductive health.

RESULTS

Delayed care has occurred for various conditions since the beginning of the COVID-19 pandemic, but the drivers for those delays have yet to be thoroughly investigated. While delayed care can be either voluntary or involuntary, the determinants of delayed care often intersect with systemic inequalities that are important to understand for pandemic responses and future preparedness.

CONCLUSION

Human biologists and anthropologists are well poised to lead the research on postpandemic population health consequences of delayed care.

The beginning and ending of a respiratory viral pandemic-lessons from the Spanish flu

The COVID‐19 pandemic goes into its third year and the world population is longing for an end to the pandemic. Computer simulations of the future development of the pandemic have wide error margins and predictions on the evolution of new viral variants of SARS‐CoV‐2 are uncertain. It is thus tempting to look into the development of historical viral respiratory pandemics for insight into the dynamic of pandemics. The Spanish flu pandemic of 1918 caused by the influenza virus H1N1 can here serve as a potential model case. Epidemiological observations on the shift of influenza mortality from very young and old subjects to high mortality in young adults delimitate the pandemic phase of the Spanish flu from 1918 to 1920. The identification and sequencing of the Spanish flu agent allowed following the H1N1 influenza virus after the acute pandemic phase. During the 1920s H1N1 influenza virus epidemics with substantial mortality were still observed. As late as 1951, H1N1 strains of high virulence evolved but remained geographically limited. Until 1957, the H1N1 virus evolved by accumulation of mutations (‘antigenic drift’) and some intratypic reassortment. H1N1 viruses were then replaced by the pandemic H2N2 influenza virus from 1957, which was in 1968 replaced by the pandemic H3N2 influenza virus; both viruses were descendants from the Spanish flu agent but showed the exchange of entire gene segments (‘antigenic shift’). In 1977, H1N1 reappeared from an unknown source but caused only mild disease. However, H1N1 achieved again circulation in the human population and is now together with the H3N2 influenza virus an agent of seasonal influenza winter epidemics.

Archival Big Data and the Spanish Flu in Copenhagen

Purpose

The Spanish Flu 1918–1920 saw a high degree of excess mortality among young and healthy adults. The purpose of this paper is a further exploration of the hypothesis that high mortality risk during The Spanish Flu in Copenhagen was associated with early life exposure to The Russian Flu 1889–1892.

Design/methodology/approach

Based on 37,000 individual-level death records in a new unique database from The Copenhagen City Archives combined with approximate cohort-specific population totals interpolated from official censuses of population, the author compiles monthly time series on all-cause mortality rates 1916–1922 in Copenhagen by gender and one-year birth cohorts. The author then analyses birth cohort effects on mortality risk during The Spanish Flu using regression analysis.

Findings

The author finds support for hypotheses relating early life exposure to The Russian Flu to mortality risk during The Spanish Flu. Some indications of possible gender heterogeneity during the first wave of The Spanish Flu – not found in previous studies – should be a topic for future research based on data from other countries.

Originality/value

Due to lack of individual-level death records with exact dates of birth and death, previous studies on The Spanish Flu in Denmark and many other countries have relied on data with lower birth cohort resolutions than the one-year birth cohorts used in this study. The analysis in this paper illustrates how archival Big Data can be used to gain new insights in studies on historical pandemics.

Prevalence and risk indicators of first-wave COVID-19 among oral health-care workers: A French epidemiological survey

BACKGROUND

Previous studies have highlighted the increased risk of contracting the COVID-19 for health-care workers and suggest that oral health-care workers may carry the greatest risk. Considering the transmission route of the SARS-CoV-2 infection, a similar increased risk can be hypothesized for other respiratory infections. However, no study has specifically assessed the risk of contracting COVID-19 within the dental profession.

METHODS

An online survey was conducted within a population of French dental professionals between April 1 and April 29, 2020. Univariable and multivariable logistic regression analyses were performed to explore risk indicators associated with laboratory-confirmed COVID-19 and COVID-19-related clinical phenotypes (i.e. phenotypes present in 15% or more of SARS-CoV-2-positive cases).

RESULTS

4172 dentists and 1868 dental assistants responded to the survey, representing approximately 10% of French oral health-care workers. The prevalence of laboratory-confirmed COVID-19 was 1.9% for dentists and 0.8% for dental assistants. Higher prevalence was found for COVID-19-related clinical phenotypes both in dentists (15.0%) and dental assistants (11.8%). Chronic kidney disease and obesity were associated with increased odds of laboratory-confirmed COVID-19, whereas working in a practice limited to endodontics was associated with decreased odds. Chronic obstructive pulmonary disease, use of public transportation and having a practice limited to periodontology were associated with increased odds of presenting a COVID-19-related clinical phenotype. Moreover, changes in work rhythm or clinical practice were associated with decreased odds of both outcomes.

CONCLUSIONS

Although oral health-care professionals were surprisingly not at higher risk of COVID-19 than the general population, specific risk indicators could exist, notably among high aerosol-generating dental subspecialties such as periodontology. Considering the similarities between COVID-19-related clinical phenotypes other viral respiratory infections, lessons can be learned from the COVID-19 pandemic regarding the usefulness of equipping and protecting oral health-care workers, notably during seasonal viral outbreaks, to limit infection spread.

IMPACT

Results from this study may provide important insights for relevant health authorities regarding the overall infection status of oral health-care workers in the current pandemic and draw attention to particular at-risk groups, as illustrated in the present study. Protecting oral health-care workers could be an interesting public health strategy to prevent the resurgence of COVID-19 and/or the emergence of new pandemics.

Comparative review of respiratory diseases caused by coronaviruses and influenza A viruses during epidemic season

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to sweep the world, causing infection of millions and death of hundreds of thousands. The respiratory disease that it caused, COVID-19 (stands for coronavirus disease in 2019), has similar clinical symptoms with other two CoV diseases, severe acute respiratory syndrome and Middle East respiratory syndrome (SARS and MERS), of which causative viruses are SARS-CoV and MERS-CoV, respectively. These three CoVs resulting diseases also share many clinical symptoms with other respiratory diseases caused by influenza A viruses (IAVs). Since both CoVs and IAVs are general pathogens responsible for seasonal cold, in the next few months, during the changing of seasons, clinicians and public heath may have to distinguish COVID-19 pneumonia from other kinds of viral pneumonia. This is a discussion and comparison of the virus structures, transmission characteristics, clinical symptoms, diagnosis, pathological changes, treatment and prevention of the two kinds of viruses, CoVs and IAVs. It hopes to provide information for practitioners in the medical field during the epidemic season.

Sex- and age-based differences in mortality during the 1918 influenza pandemic on the island of Newfoundland

OBJECTIVES

Our aim was to understand sex- and age-based differences in mortality during the 1918 influenza pandemic on the island of Newfoundland. The pandemic's impact on different age groups has been the focus of other research, but sex-based differences in mortality are rarely considered. Aspects of social organization, labor patterns, and social behaviors that contribute to mortality between males and females at all ages are used to explain observed mortality patterns.

METHODS

Recorded pneumonia and influenza deaths on the island (n = 1871) were used to calculate cause-specific death rates and to evaluate differences in sex-based mortality. Mortality levels in 17 districts and four regions (Avalon, North, South, and West) were compared using standardized mortality ratios (SMRs). A logistic regression model was fit to determine in which regions sex-based mortality could be predicted using age and region as interactive predictors.

RESULTS

Differences in sex-based mortality varied across regions; they were not significant for the aggregate population. SMRs were also variable, with no significant sex-based differences. Sex-based differences were highly variable within regions. Results from a logistic regression analysis suggest that females in the South region may have experienced a higher probability of death than other island residents.

CONCLUSIONS

Mortality analysis for aggregate populations homogenizes important epidemiologic patterns. Men and women did not experience the 1918 influenza pandemic in the same way, and by analyzing data at the regional and district geographic levels, patterns emerge that can be explained by the economies and social organization of the people who lived there.

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.

Intensive care unit surveillance of influenza infection in France: the 2009/10 pandemic and the three subsequent seasons

During the 2009/10 pandemic, a national surveillance system for severe influenza cases was set up in France. We present results from the system's first four years. All severe influenza cases admitted to intensive care units (ICU) were reported to the Institut de Veille Sanitaire using a standardised form: data on demographics, immunisation and virological status, risk factors, severity (e.g. acute respiratory distress syndrome (ARDS) onset, mechanical ventilation, extracorporeal life support) and outcome. Multivariate analysis was performed to identify factors associated with ARDS and death. The number of confirmed influenza cases varied from 1,210 in 2009/10 to 321 in 2011/12. Most ICU patients were infected with A(H1N1)pdm09, except during the 2011/12 winter season when A(H3N2)-related infections predominated. Patients' characteristics varied according to the predominant strain. Based on multivariate analysis, risk factors associated with death were age ≥ 65 years, patients with any of the usual recommended indications for vaccination and clinical severity. ARDS occurred more frequently in patients who were middle-aged (36–55 years), pregnant, obese, or infected with A(H1N1)pdm09. Female sex and influenza vaccination were protective. These data confirm the persistent virulence of A(H1N1)pdm09 after the pandemic and the heterogeneity of influenza seasons, and reinforce the need for surveillance of severe influenza cases.

Demographic buffering: titrating the effects of birth rate and imperfect immunity on epidemic dynamics

Host demography can alter the dynamics of infectious disease. In the case of perfectly immunizing infections, observations of strong sensitivity to demographic variation have been mechanistically explained through analysis of the susceptible–infected–recovered (SIR) model that assumes lifelong immunity following recovery from infection. When imperfect immunity is incorporated into this framework via the susceptible–infected–recovered–susceptible (SIRS) model, with individuals regaining full susceptibility following recovery, we show that rapid loss of immunity is predicted to buffer populations against the effects of demographic change. However, this buffering is contrary to the dependence on demography recently observed for partially immunizing infections such as rotavirus and respiratory syncytial virus. We show that this discrepancy arises from a key simplification embedded in the SIR(S) framework, namely that the potential for differential immune responses to repeat exposures is ignored. We explore the minimum additional immunological information that must be included to reflect the range of observed dependencies on demography. We show that including partial protection and lower transmission following primary infection is sufficient to capture more realistic reduced levels of buffering, in addition to changes in epidemic timing, across a range of partially and fully immunizing infections. Furthermore, our results identify key variables in this relationship, including R₀.

Insights from unusual aspects of the 1918 influenza pandemic

The 1918 influenza pandemic was the most lethal single event in modern history. Besides its mortality the 1918 pandemic was unusual for several reasons. It preferentially killed young adults from 20 to 40 y with a peak mortality at age 28 y. Mortality was highly variable with death rates varying by at least 10 fold within similar groups of citizens, soldiers, cities and islands. Secondary bacterial pneumonia following influenza was the overwhelming cause of death and not viral pneumonitis or acute lung injury. Clinical expressions of the 1918 pandemic were unusual with bleeding into the respiratory tree including epistaxis and dark blue cyanotic skin. The 1918 influenza virus apparently ceased circulation in the human population in the early 1920s but continued to evolve in pigs. Immunizations using viruses from 1918 and 2009 can cross-protect laboratory animals even though the human mortality outcomes were very different between the first pandemics of the 20th and 21st centuries. Unusual aspects of historical epidemics may help to reconstruct what actually occurred in 1918 and thus better prepare for the next pandemic.

WITHDRAWN: Intense Seasonal A/H1N1 Influenza in Mexico, Winter 2013-2014

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/10.1016/j.arcmed.2014.11.005. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Intense seasonal A/H1N1 influenza in Mexico, winter 2013-2014

BACKGROUND AND AIMS

A recrudescent wave of pandemic influenza A/H1N1 affected Mexico during the winter of 2013-2014 following a mild 2012-2013 A/H3N2 influenza season.

METHODS

We compared the demographic and geographic characteristics of hospitalizations and inpatient deaths for severe acute respiratory infection (SARI) and laboratory-confirmed influenza during the 2013-2014 influenza season compared to previous influenza seasons, based on a large prospective surveillance system maintained by the Mexican Social Security health care system.

RESULTS

A total of 14,236 SARI hospitalizations and 1,163 inpatient deaths (8.2%) were reported between October 1, 2013 and March 31, 2014. Rates of laboratory-confirmed A/H1N1 hospitalizations and deaths were significantly higher among individuals aged 30-59 years and lower among younger age groups for the 2013-2014 A/H1N1 season compared to the previous A/H1N1 season in 2011-2012 (χ(2) test, p <0.001). The reproduction number for the winter 2013-2014 influenza season in central Mexico was estimated at 1.3-1.4, in line with that reported for the 2011-2012 A/H1N1 season but lower than during the initial waves of pandemic A/H1N1 activity in 2009.

CONCLUSIONS

We documented a substantial increase in the number of A/H1N1-related hospitalizations and deaths during the period from October 2013-March 2014 in Mexico and a proportionate shift of severe disease to middle-aged adults, relative to the preceding A/H1N1 2011-2012 season. In the absence of clear antigenic drift in globally circulating A/H1N1 viruses in the post-2009 pandemic period, the gradual change in the age distribution of A/H1N1 infections observed in Mexico suggests a slow build-up of immunity among younger populations, reminiscent of the age profile of past pandemics.

Spatial-temporal excess mortality patterns of the 1918-1919 influenza pandemic in Spain

Background

The impact of socio-demographic factors and baseline health on the mortality burden of seasonal and pandemic influenza remains debated. Here we analyzed the spatial-temporal mortality patterns of the 1918 influenza pandemic in Spain, one of the countries of Europe that experienced the highest mortality burden.

Methods

We analyzed monthly death rates from respiratory diseases and all-causes across 49 provinces of Spain, including the Canary and Balearic Islands, during the period January-1915 to June-1919. We estimated the influenza-related excess death rates and risk of death relative to baseline mortality by pandemic wave and province. We then explored the association between pandemic excess mortality rates and health and socio-demographic factors, which included population size and age structure, population density, infant mortality rates, baseline death rates, and urbanization.

Results

Our analysis revealed high geographic heterogeneity in pandemic mortality impact. We identified 3 pandemic waves of varying timing and intensity covering the period from Jan-1918 to Jun-1919, with the highest pandemic-related excess mortality rates occurring during the months of October-November 1918 across all Spanish provinces. Cumulative excess mortality rates followed a south–north gradient after controlling for demographic factors, with the North experiencing highest excess mortality rates. A model that included latitude, population density, and the proportion of children living in provinces explained about 40% of the geographic variability in cumulative excess death rates during 1918–19, but different factors explained mortality variation in each wave.

Conclusions

A substantial fraction of the variability in excess mortality rates across Spanish provinces remained unexplained, which suggests that other unidentified factors such as comorbidities, climate and background immunity may have affected the 1918–19 pandemic mortality rates. Further archeo-epidemiological research should concentrate on identifying settings with combined availability of local historical mortality records and information on the prevalence of underlying risk factors, or patient-level clinical data, to further clarify the drivers of 1918 pandemic influenza mortality.

Substantial Morbidity and Mortality Associated with Pandemic A/H1N1 Influenza in Mexico, Winter 2013-2014: Gradual Age Shift and Severity

BACKGROUND

A recrudescent wave of pandemic influenza A/H1N1 is underway in Mexico in winter 2013-14, following a mild 2012-13 A/H3N2 influenza season. Mexico previously experienced several waves of pandemic A/H1N1 activity in spring, summer and fall 2009 and winter 2011-2012, with a gradual shift of influenza-related hospitalizations and deaths towards older ages. Here we describe changes in the epidemiology of the 2013-14 A/H1N1 influenza outbreak, relative to previous seasons dominated by the A/H1N1 pandemic virus. The analysis is intended to guide public health intervention strategies in near real time.

METHODS

We analyzed demographic and geographic data on hospitalizations with severe acute respiratory infection (SARI), laboratory-confirmed A/H1N1 influenza hospitalizations, and inpatient deaths, from a large prospective surveillance system maintained by the Mexican Social Security medical system during 01-October 2013 to 31-Jan 2014. We characterized the age and regional patterns of influenza activity relative to the preceding 2011-2012 A/H1N1 influenza epidemic. We also estimated the reproduction number (R) based on the growth rate of daily case incidence by date of symptoms onset.

RESULTS

A total of 7,886 SARI hospitalizations and 529 inpatient-deaths (3.2%) were reported between 01-October 2013 and 31-January 2014 (resulting in 3.2 laboratory-confirmed A/H1N1 hospitalizations per 100,00 and 0.52 laboratory-confirmed A/H1N1-positive deaths per 100,000). The progression of daily SARI hospitalizations in 2013-14 exceeded that observed during the 2011-2012 A/H1N1 epidemic. The mean age of laboratory-confirmed A/H1N1 patients in 2013-14 was 41.1 y (SD=20.3) for hospitalizations and 49.2 y (SD=16.7) for deaths. Rates of laboratory-confirmed A/H1N1 hospitalizations and deaths were significantly higher among individuals aged 30-59 y and lower among younger age groups for the ongoing 2013-2014 epidemic, compared to the 2011-12 A/H1N1 epidemic (Chi-square test, P<0.001). The reproduction number of the winter 2013-14 wave in central Mexico was estimated at 1.3-1.4 which is slightly higher than that reported for the 2011-2012 A/H1N1 epidemic.

CONCLUSIONS

We have documented a substantial and ongoing increase in the number of A/H1N1-related hospitalizations and deaths during the period October 2013-January 2014 and a proportionate shift of severe disease to middle aged adults, relative to the preceding A/H1N1 2011-2012 epidemic in Mexico. In the absence of clear antigenic drift in globally circulating A/H1N1 viruses in the post-pandemic period, the gradual change in the age distribution of A/H1N1 infections observed in Mexico suggests a slow build-up of immunity among younger populations, reminiscent of the age profile of past pandemics.

The Spanish flu in Uppsala, clinical and epidemiological impact of the influenza pandemic 1918-1919 on a Swedish county

Introduction and aim

The Spanish flu reached Sweden in June 1918, and at least one-third of the population (then 5.8 million) became infected. Some 34,500 persons (5.9 per 1,000 people) died from influenza during the first year of the pandemic (when acute pneumonia is included, the number of deaths rose to 7.1 per 1,000 people). In this historical look back at the pandemic, our aim was to review the epidemiological impact on the Swedish county of Uppsala, the clinical outcomes and the economic impact on the regional hospital; a relevant backgound to consider the impact of a future virulent pandemic. We also focused on how the pandemic was perceived by the medical community and by health care authorities.

Methods

Health care reports, statistics, daily newspapers, medical journals, and records of patients treated for influenza at the Uppsala Academic Hospital from July 1918 to June 1919 were included in our review.

Results

An influenza related mortality rate of 693 persons (5.1 per 1,000 people) was reported in the Uppsala region from 1918–1919; from July 1918 to June 1919, 384 patients were treated for influenza at the Uppsala Academic Hospital. The first wave peaked in November 1918 with case fatality rates up to 30%; a second wave peaked in April 1919 with a lower rate of mortality. Of the patients treated, a total of 66 died. Of these, 60% were 20–29 years of age, and 85% were less than 40 years old. Autopsy reports revealed pneumonia in 89% of the cases; among these, 47% were hemorrhagic, 18% were bilateral, and 45% had additional extrapulmonary organ involvement. Signs of severe viral disease were documented, but secondary bacterial disease was the primary cause of death in the majority of cases.

Conclusion

The epidemiologic and pathologic results were in accordance with other publications of this time period. The costs of running the hospital doubled from 1917 to 1920 and then reversed by 45%. Today, an influenza pandemic of the same virulence would paralyze health care systems and result in extremely high financial costs and rates of mortality.

Recrudescent Wave of A/H1N1pdm09 Influenza Viruses in Winter 2012-2013 in Kashmir, India

Some parts of world, including India observed a recrudescent wave of influenza A/H1N1pdm09 in 2012. We undertook a study to examine the circulating influenza strains, their clinical association and antigenic characteristics to understand the recrudescent wave of A/H1N1pdm09 from November 26, 2012 to Feb 28, 2013 in Kashmir, India. Of the 751 patients (545 outpatient and 206 hospitalized) presenting with acute respiratory infection at a tertiary care hospital in Srinagar; 184 (24.5%) tested positive for influenza. Further type and subtype analysis revealed that 106 (58%) were influenza A (H1N1pdm09 =105, H3N2=1) and 78 (42%) were influenza B. The influenza positive cases had a higher frequency of chills, nasal discharge, sore throat, body aches and headache, compared to influenza negative cases. Of the 206 patients hospitalized for pneumonia/acute respiratory distress syndrome or an exacerbation of an underlying lung disease, 34 (16.5%) tested positive for influenza (22 for H1N1pdm09, 11 for influenza B). All influenza-positive patients received oseltamivir and while most patients responded well to antiviral therapy and supportive care, 6 patients (4 with H1N1pdm09 and 2 with influenza B) patients died of progressive respiratory failure and multi-organ dysfunction. Following a period of minimal circulation, H1N1pdm09 re-emerged in Kashmir in 2012-2013, causing serious illness and fatalities. As such the healthcare administrators and policy planners need to be wary and monitor the situation closely.

Seroprevalence of influenza A(H1N1)pdm09 virus antibody, England, 2010 and 2011

The intense influenza activity in England during the 2010-11 winter resulted from a combination of factors. Population-based seroepidemiology confirms that the third wave of influenza A(H1N1)pdm09 virus circulation was associated with a shift in age groups affected, with the highest rate of infection in young adults.

Burden and characteristics of influenza A and B in Danish intensive care units during the 2009/10 and 2010/11 influenza seasons

Influenza surveillance in Danish intensive care units (ICUs) was performed during the 2009/10 and 2010/11 influenza seasons to monitor the burden on ICUs. All 44 Danish ICUs reported aggregate data for incidence and point prevalence, and case-based demographical and clinical parameters. Additional data on microbiological testing, vaccination and death were obtained from national registers. Ninety-six patients with influenza A(H1N1)pdm09 were recorded in 2009/10; 106 with influenza A and 42 with influenza B in 2010/11. The mean age of influenza A patients was higher in 2010/11 than in 2009/10, 53 vs. 44 years (P = 0·004). No differences in other demographic and clinical parameters were detected between influenza A and B patients. In conclusion, the number of patients with severe influenza was higher in Denmark during the 2010/11 than the 2009/10 season with a shift towards older age groups in influenza A patients. Influenza B caused severe illness and needs consideration in clinical and public health policy.

Influenza pandemics of 1918 and 2009: a comparative account

The 2009 influenza pandemic A(H1N1)pdm09 of swine origin and the continued circulation of highly pathogenic avian H5N1 strain in humans are stark reminders of the unpredictable nature of the influenza virus. Experiences from the 1918 and 20th century influenza pandemics helped immensely in the preparation of a better response for A(H1N1)pdm09. The explosive pattern of the 1918 pandemic makes it a benchmark for pandemic planning and preparedness today. Its similarities with the 2009 pandemic makes it even more intriguing, and it is a great surprise that the two strains, separated by a period of 91 years, share such similar features. This review is an attempt to summarize the literature describing the important features of the 1918 and 2009 pandemics. This may provide a better understanding for the early detection and control of influenza pandemics in the future.

The age distribution of mortality due to influenza: pandemic and peri-pandemic

BACKGROUND

Pandemic influenza is said to 'shift mortality' to younger age groups; but also to spare a subpopulation of the elderly population. Does one of these effects dominate? Might this have important ramifications?

METHODS

We estimated age-specific excess mortality rates for all-years for which data were available in the 20th century for Australia, Canada, France, Japan, the UK, and the USA for people older than 44 years of age. We modeled variation with age, and standardized estimates to allow direct comparison across age groups and countries. Attack rate data for four pandemics were assembled.

RESULTS

For nearly all seasons, an exponential model characterized mortality data extremely well. For seasons of emergence and a variable number of seasons following, however, a subpopulation above a threshold age invariably enjoyed reduced mortality. 'Immune escape', a stepwise increase in mortality among the oldest elderly, was observed a number of seasons after both the A(H2N2) and A(H3N2) pandemics. The number of seasons from emergence to escape varied by country. For the latter pandemic, mortality rates in four countries increased for younger age groups but only in the season following that of emergence. Adaptation to both emergent viruses was apparent as a progressive decrease in mortality rates, which, with two exceptions, was seen only in younger age groups. Pandemic attack rate variation with age was estimated to be similar across four pandemics with very different mortality impact.

CONCLUSIONS

In all influenza pandemics of the 20th century, emergent viruses resembled those that had circulated previously within the lifespan of then-living people. Such individuals were relatively immune to the emergent strain, but this immunity waned with mutation of the emergent virus. An immune subpopulation complicates and may invalidate vaccine trials. Pandemic influenza does not 'shift' mortality to younger age groups; rather, the mortality level is reset by the virulence of the emerging virus and is moderated by immunity of past experience. In this study, we found that after immune escape, older age groups showed no further mortality reduction, despite their being the principal target of conventional influenza vaccines. Vaccines incorporating variants of pandemic viruses seem to provide little benefit to those previously immune. If attack rates truly are similar across pandemics, it must be the case that immunity to the pandemic virus does not prevent infection, but only mitigates the consequences.

Age- and sex-specific mortality associated with the 1918-1919 influenza pandemic in Kentucky

Background. The reasons for the unusual age-specific mortality patterns of the 1918–1919 influenza pandemic remain unknown. Here we characterize pandemic-related mortality by single year of age in a unique statewide Kentucky data set and explore breakpoints in the age curves.

Methods. Individual death certificates from Kentucky during 1911–1919 were abstracted by medically trained personnel. Pandemic-associated excess mortality rates were calculated by subtracting observed rates during pandemic months from rates in previous years, separately for each single year of age and by sex.

Results. The age profile of excess mortality risk in fall 1918 was characterized by a maximum among infants, a minimum at ages 9–10 years, a maximum at ages 24–26 years, and a second minimum at ages 56–59 years. The excess mortality risk in young adults had been greatly attenuated by winter 1919. The age breakpoints of mortality risk did not differ between males and females.

Conclusions. The observed mortality breakpoints in male and female cohorts born during 1859–1862, 1892–1894, and 1908–1909 did not coincide with known dates of historical pandemics. The atypical age mortality patterns of the 1918–1919 pandemic cannot be explained by military crowding, war-related factors, or prior immunity alone and likely result from a combination of unknown factors.

Epidemiological characterization of a fourth wave of pandemic A/H1N1 influenza in Mexico, winter 2011-2012: age shift and severity

BACKGROUND AND AIMS

A substantial recrudescent wave of pandemic influenza A/H1N1 affected the Mexican population from December 1, 2011-March 20, 2012 following a 2-year period of sporadic transmission.

METHODS

We analyzed demographic and geographic data on all hospitalizations with severe acute respiratory infection (SARI) and laboratory-confirmed A/H1N1 influenza, and inpatient deaths, from a large prospective surveillance system maintained by a Mexican social security medical system during April 1, 2009-March 20, 2012. We also estimated the reproduction number (R) based on the growth rate of the daily case incidence by date of symptoms onset.

RESULTS

A total of 7569 SARI hospitalizations and 443 in-patient deaths (5.9%) were reported between December 1, 2011, and March 20, 2012 (1115 A/H1N1-positive inpatients and 154 A/H1N1-positive deaths). The proportion of laboratory-confirmed A/H1N1 hospitalizations and deaths was higher among subjects ≥60 years of age (χ(2) test, p <0.0001) and lower among younger age groups (χ(2) test, p <0.04) for the 2011-2012 pandemic wave compared to the earlier waves in 2009. The reproduction number of the winter 2011-2012 wave in central Mexico was estimated at 1.2-1.3, similar to that reported for the fall 2009 wave, but lower than that of spring 2009.

CONCLUSIONS

We documented a substantial increase in the number of SARI hospitalizations during the period December 2011-March 2012 and an older age distribution of laboratory-confirmed A/H1N1 influenza hospitalizations and deaths relative to 2009 A/H1N1 pandemic patterns. The gradual change in the age distribution of A/H1N1 infections in the post-pandemic period is consistent with a build-up of immunity among younger populations.

Differential pathological and immune responses in newly weaned ferrets are associated with a mild clinical outcome of pandemic 2009 H1N1 infection

Young children are typically considered a high-risk group for disease associated with influenza virus infection. Interestingly, recent clinical reports suggested that young children were the smallest group of cases with severe pandemic 2009 H1N1 (H1N1pdm) influenza virus infection. Here we established a newly weaned ferret model for the investigation of H1N1pdm infection in young age groups compared to adults. We found that young ferrets had a significantly milder fever and less weight loss than adult ferrets, which paralleled the mild clinical symptoms in the younger humans. Although there was no significant difference in viral clearance, disease severity was associated with pulmonary pathology, where newly weaned ferrets had an earlier pathology improvement. We examined the immune responses associated with protection of the young age group during H1N1pdm infection. We found that interferon and regulatory interleukin-10 responses were more robust in the lungs of young ferrets. In contrast, myeloperoxidase and major histocompatibility complex responses were persistently higher in the adult lungs; as well, the numbers of inflammation-prone granulocytes were highly elevated in the adult peripheral blood. Importantly, we observed that H1N1pdm infection triggered formation of lung structures that resembled inducible bronchus-associated lymphoid tissues (iBALTs) in young ferrets which were associated with high levels of homeostatic chemokines CCL19 and CXCL13, but these were not seen in the adult ferrets with severe disease. These results may be extrapolated to a model of the mild disease seen in human children. Furthermore, these mechanistic analyses provide significant new insight into the developing immune system and effective strategies for intervention and vaccination against respiratory viruses.

Recrudescent wave of pandemic A/H1N1 influenza in Mexico, winter 2011-2012: Age shift and severity

BACKGROUND

A substantial recrudescent wave of pandemic influenza A/H1N1 that began in December 2011 is ongoing and has not yet peaked in Mexico, following a 2-year period of sporadic transmission. Mexico previously experienced three pandemic waves of A/H1N1 in 2009, associated with higher excess mortality rates than those reported in other countries, and prompting a large influenza vaccination campaign. Here we describe changes in the epidemiological patterns of the ongoing 4th pandemic wave in 2011-12, relative to the earlier waves in 2009. The analysis is intended to guide public health intervention strategies in near real time.

METHODS

We analyzed demographic and geographic data on all hospitalizations with acute respiratory infection (ARI) and laboratory-confirmed A/H1N1 influenza, and inpatient deaths, from a large prospective surveillance system maintained by the Mexican Social Security medical system during 01-April 2009 to 10-Feb 2012. We characterized the age and regional patterns of A/H1N1-positive hospitalizations and inpatient-deaths relative to the 2009 A/H1N1 influenza pandemic. We also estimated the reproduction number (R) based on the growth rate of the daily case incidence by date of symptoms onset.

RESULTS

A total of 5,795 ARI hospitalizations and 186 inpatient-deaths (3.2%) were reported between 01-December 2011 and 10-February 2012 (685 A/H1N1-positive inpatients and 75 A/H1N1-positive deaths). The nationwide peak of daily ARI hospitalizations in early 2012 has already exceeded the peak of ARI hospitalizations observed during the major fall pandemic wave in 2009. The mean age was 34.3 y (SD=21.3) among A/H1N1 inpatients and 43.5 y (SD=21) among A/H1N1 deaths in 2011-12. The proportion of laboratory-confirmed A/H1N1 hospitalizations and deaths was higher among seniors >=60 years of age (Chi-square test P<0.001) and lower among younger age groups (Chi-square test, P<0.03) for the 2011-2012 pandemic wave, compared to the earlier waves in 2009. The reproduction number of the winter 2011-12 wave in central Mexico was estimated at 1.2-1.3, similar to that reported for the fall 2009 wave, but lower than that of spring 2009.

CONCLUSIONS

We have documented a substantial and ongoing increase in the number of ARI hospitalizations during the period December 2011-February 2012 and an older age distribution of laboratory-confirmed A/H1N1 influenza hospitalizations and deaths, relative to 2009 A/H1N1 pandemic patterns. The gradual change in the age distribution of A/H1N1 infections in the post-pandemic period is reminiscent of historical pandemics and indicates either a gradual drift in the A/H1N1 virus, and/or a build-up of immunity among younger populations.

Mortality burden of the A/H1N1 pandemic in Mexico: a comparison of deaths and years of life lost to seasonal influenza

BACKGROUND

The mortality burden of the 2009 A/H1N1 influenza pandemic remains controversial, in part because of delays in reporting of vital statistics that are traditionally used to measure influenza-related excess mortality. Here, we compare excess mortality rates and years of life lost (YLL) for pandemic and seasonal influenza in Mexico and evaluate laboratory-confirmed death reports.

METHODS

Monthly age- and cause-specific death rates from January 2000 through April 2010 and population-based surveillance of influenza virus activity were used to estimate excess mortality and YLL in Mexico. Age-stratified laboratory-confirmed A/H1N1 death reports were obtained from an active surveillance system covering 40% of the population.

RESULTS

The A/H1N1 pandemic was associated with 11.1 excess all-cause deaths per 100,000 population and 445,000 YLL during the 3 waves of virus activity in Mexico, April-December 2009. The pandemic mortality burden was 0.6-2.6 times that of a typical influenza season and lower than that of the severe 2003-2004 influenza epidemic. Individuals aged 5-19 and 20-59 years were disproportionately affected relative to their experience with seasonal influenza. Laboratory-confirmed deaths captured 1 of 7 pandemic excess deaths overall but only 1 of 41 deaths in persons >60 years of age in 2009. A recrudescence of excess mortality was observed in older persons during winter 2010, in a period when influenza and respiratory syncytial virus cocirculated.

CONCLUSIONS

Mexico experienced higher 2009 A/H1N1 pandemic mortality burden than other countries for which estimates are available. Further analyses of detailed vital statistics are required to assess geographical variation in the mortality patterns of this pandemic.