At least one in five people, including half of schoolchildren, were infected with swine flu during the first year of the pandemic in 2009, according to data from 19 countries.
It is thought the virus killed 200,000 people around the world.
A World Health Organization-led study looked for evidence of the body's immune system fighting the virus.
It showed large numbers of people had been infected, although not all would have developed full-blown flu.
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Swine flu infected 'fifth of people'swine flu
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