2009 H1N1 Pandemic Infected 1 in 5 People, Experts Estimate

Troy Brown

January 29, 2013

At least 20% of all people, and almost half (46%) of all children aged 5 to 19 years, were infected with H1N1 influenza during the first year of the 2009 pandemic, according to a meta-analysis of data from 27 seroepidemiological studies. The studies analyzed approximately 90,000 serologic samples from 19 countries including the United States, United Kingdom, Australia, Canada, China, and India.

Maria D. Van Kerhove, PhD, a liaison between the Global Influenza Programme at the World Health Organization in Geneva, Switzerland, and the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London in the United Kingdom, and colleagues published their findings in an article published online January 21 in Influenza and Other Respiratory Viruses.

Before the pandemic, the overall age-adjusted prevalence of elevated cross-reactive H1N1pandemic (pdm) antibodies was 5% (95% confidence interval [CI], 3% - 7%). The prevalence increased as individuals aged (0 - 4 years old, 1% [95% CI, 0.3% - 4%]; 5 - 19 years old, 4% [95% CI, 1% - 9%]; 20 - 44 years old, 5% [95% CI, 3% - 8%]; and 45 - 64 years old, 5% [95% CI, 2% - 9%]). Prevalence was the highest in individuals at least 65 years old (14%; 95% CI, 8% - 24%).

The researchers calculated the difference between pre- and postpandemic seroprevalence to estimate age-specific cumulative incidence. Data were available from 11 countries and 12 studies. Overall, the age-adjusted cumulative incidence of H1N1pdm infection was 24% (95% CI, 20% - 27%).

There was significant variation between the age groups, with the highest age-specific incidence found among children aged 5 to 19 years (46%; 95% CI, 36% - 56%) and 0 to 4 years (37%; 95% CI, 30% - 44%). Lower incidence rates were found in participants aged 20 years and older (20 - 44 years old, 20% [95% CI, 13% - 26%]; 45 - 64 years, 14% [95% CI, 9% - 20%]). The incidence was lowest in individuals at least 65 years old (11%; 95% CI, 5% - 18%).

Older individuals may have developed immunity to the 2009 virus as a result of exposure to multiple influenza viruses during the course of their lifetimes.

The researchers pooled data from 10 studies that included 9 countries (n sera = 52,479) and found an overall age-adjusted H1N1pdm seroprevalence of 32% (95% CI, 26% - 39%). For the most part, seroprevalence decreased with age after age 5 years, but this was not statistically significant.

"Assuming that the cumulative incidence in the countries included in our studies is similar to the rest of the world for which no [or] little data exist and if the global mortality estimates produced by two research groups are confirmed by other studies, this would place the [case fatality ratio] for H1N1pdm at < 0.02%," the authors write.

"Knowing the proportion of the population infected in different age groups and the proportion of those infected who died will help public health decision-makers plan for and respond to pandemics. This information will be used to quantify severity and develop mathematical models to predict how flu outbreaks spread and what effect different interventions may have," said senior author Anthony Mounts, MD, from the World Health Organization, in a press release.

Support was provided by the Medical Research Council UK, the Bill and Melinda Gates Foundation, and a MRC Population Health Scientist Fellowship.

Influenza Other Respi Viruses. Published online January 21, 2013. Abstract