Abstract
BackgroundSeveral studies have shown a relatively high mortality rate among young people infected by the 2009 pandemic influenza A (H1N1) virus. Here we compared the age distributions of morbidity and mortality during two seasonal influenza epidemics (H1N1 and H3N2) in France and the United States with those of the 2009 H1N1 pandemic waves in the same countries.MethodsAge-standardized ratios were used to compare the age distribution of morbidity and mortality due to influenza between the two countries and across the different years. Non parametric analysis of variance was used to compare these ratios between epidemic and pandemic influenza.ResultsAge distribution of morbidity was similar between the 2009 pandemic and seasonal epidemics due to H1N1 (p = 0.72) and H3N2 viruses (p = 0.68). In contrast, the proportion of under-60s among influenza deaths was markedly higher during the 2009 pandemic (peak <20 years) than during the seasonal epidemics (respectively p = 0.007 and p = 0.0008).ConclusionsYoung age was a principal mortality risk factor due to the 2009 H1N1 pandemic.
Highlights
Several studies have shown a relatively high mortality rate among young people infected by the 2009 pandemic influenza A (H1N1) virus
The 2009 H1N1 pandemic in the US and France resembled the seasonal epidemics of H1N1 (p = 0.72) and H3N2 influenza (p = 0.68)
The maximal relative illness ratio (RIR) values for the seasonal epidemics in France were obtained for the 5- to 9-year age group during the 1988-89 H1N1 epidemic (2.15) and the 5- to 9-year age group during the 1989-90 H3N2 epidemic (1.77)
Summary
Several studies have shown a relatively high mortality rate among young people infected by the 2009 pandemic influenza A (H1N1) virus. We compared the age distributions of morbidity and mortality during two seasonal influenza epidemics (H1N1 and H3N2) in France and the United States with those of the 2009 H1N1 pandemic waves in the same countries. Preliminary findings in Mexico and the United States suggested that people aged from 5 to 30 years were at a higher risk of infection, while those aged 30 to 50 years old were at a higher risk of death [2,3,4]. During pandemic influenza mortality is highest in younger age groups. Few comparative studies are available on age distribution of morbidity during pandemic and seasonal influenza. Studies of the 1918, 1957 and 1968 pandemics suggest that the
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