The final count: One in five people worldwide got the swine flu

About 50% of school-age children were infected, research shows

The swine flu infected at least 20% of the world's population during the year of the H1N1 pandemic, according to researchers from the World Health Organization (WHO) and Imperial College London.

For the study, published in the journal Influenza and Other Respiratory Viruses, researchers collected data from 19 countries and reviewed more than 24 studies involving 90,000 blood samples taken before, during, and after the H1N1 pandemic was declared in June 2009.

They found that between 20% and 27% of people worldwide were infected in the pandemic's first year. About 50% of school-age children were infected by the H1N1 flu, while elderly individuals were less vulnerable to H1N1 than to seasonal influenza due to prior exposure to the virus.

Altogether, the pandemic's death rate was about 0.02%, and about 200,000 individuals worldwide died.

"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" better plan for pandemics, lead author Anthony Mounts told Reuters.

In addition, the information could be used to develop mathematical models to predict how flu outbreaks spread and what effect different preventive measures—such as vaccinations and closing down schools—could have on an outbreak, Mounts said.

"[W]e know this will happen again and there is a lot of effort being put into trying to prepare now...for the next one," says study co-author Maria Van Kerkhove (Kelland, Reuters, 1/25; Gallagher, BBC News, 1/25).

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