The flu epidemic in 2009 has infected one in five people, reveals an international study. The highest rates of infection were in children, with 47 percent of those aged five to 19 showing signs of having caught the virus.
Older people were affected less, with only 11 percent of people aged 65 or older becoming infected.
The study collated results from more than two dozen research studies involving more than 90,000 blood samples collected before, during and after the pandemic.
The samples were tested for antibodies produced by the body in response to the specific flu strain that caused the pandemic, the journal Influenza and Other Respiratory Viruses reports.
The findings come from an international collaboration led by the WHO and Imperial College London, which analysed data from 19 countries, to assess the global impact of the 2009 flu pandemic, according to an Imperial statement.
Multiple exposures to previously circulating influenza viruses may have given older people some protection against the strain that emerged in 2009.
Blood samples from before the pandemic showed that 14 percent of people aged 65 or over already had antibodies that reacted to the 2009 strain.
Maria Van Kerkhove, from the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London, study co-author, said: "This study is the result of a combined effort by more than 27 research groups worldwide, who all shared their data and experience with us to help improve our understanding of the impact the pandemic had globally."
Anthony Mounts of the World Health Organization and senior author, said: "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."