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. 2010 Jun 9:10:162.
doi: 10.1186/1471-2334-10-162.

Comparative age distribution of influenza morbidity and mortality during seasonal influenza epidemics and the 2009 H1N1 pandemic

Magali Lemaitre  1 , Fabrice Carrat
Affiliations

Comparative age distribution of influenza morbidity and mortality during seasonal influenza epidemics and the 2009 H1N1 pandemic

Magali Lemaitre et al. BMC Infect Dis. .

Abstract

Background: Several 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.

Methods: Age-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.

Results: Age 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).

Conclusions: Young age was a principal mortality risk factor due to the 2009 H1N1 pandemic.

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Figures

Figure 1
Figure 1
Relative illness ratio (RIR) across age groups. The RIR is the ratio of the percentage of sick persons in a given age group to the percentage of the general population belonging to the same age group. The age distribution of morbidity were established from data based on cohorts of 6,000 patients during the 1978-79 seasonal epidemic and 59,785 patients during the 1989-90 seasonal epidemic in the US. The age distribution of morbidity were obtained from the report of 18,839 (1988-89) and 27,054 (1989-90) cases in France. Pandemic age-distribution were obtained from 11,485 (France) and 658,078 (US) cases.
Figure 2
Figure 2
Relative mortality ratio (RMR) across age groups. The RMR is the ratio between the percentage of influenza deaths in a given age group and the percentage of all-cause deaths in the same age group. The age distribution of mortality due to influenza were established from the report of 368 influenza deaths during the 1978-79 seasonal epidemic and 1,926 influenza deaths during the 1989-90 seasonal epidemic in the US. The age distribution of mortality due to influenza were obtained from the report of 508 (1988-89) and 1,990 (1989-90) influenza deaths in France. Pandemic age-distribution were obtained from 308 (France) and 6,317 (US) influenza deaths.
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