A World Health Organization-led study analyzed data from 19 countries, searching for evidence of the body's immune system fighting the H1N1 virus. Researchers examined more than 90,000 blood samples collected before and during the pandemic from countries across the globe.
They said there was evidence of wide-spread infection, although they noted not all infected people would have developed full-on flu symptoms.
First detected in Mexico in 2009, H1N1 spread quickly around the world.
Fewer than two in every 10,000 people infected died during the pandemic, the researchers said.
"However, those that did die are much younger than in seasonal flu so the years of life lost will be much more," Dr. Maria Van Kerkhove from Imperial College London told the BBC.
"The figures drive home how incredibly infectious the virus is."
John Oxford, a virology expert at Queen Mary, University of London, agreed.
"It was the busiest virus on the block and it displaced other influenza viruses -- it was the only virus in town," he said.
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