The H1N1 virus in the 2009-2010 flu pandemic infected more than a fifth of world's population, new estimates have revealed.
The study confirms warnings that so-called swine flu was highly contagious. It also estimates that the flu's lethality -- as previously reported -- was low.
Between 20 and 27 percent of people were infected by the virus, the investigators reported in a specialist journal, Influenza and Other Respiratory Viruses.
Children aged five to 19 had the highest rates of infection, accounting for 47 percent of the total. Older people aged 65 and above accounted for 11 percent.
The probe aims to give the most complete picture of the pandemic to help future preparations for flu outbreaks.
The 16-month episode sparked a surge in demand for vaccines, prompting critics to accuse health watchdogs of scare-mongering.
By the time the pandemic was officially over in August 2010, countries had notified the UN's World Health Organisation (WHO) of less than one million infections and around 18,500 deaths, but this has always been known to be a small fraction of the toll.
The new study is based on data from 27 pieces of research that looked for traces of antibodies to H1N1 in more than 90,000 blood samples collected in 19 countries. Presence of the antibodies is a telltale of infection, but does not in itself mean the individual fell ill.
The investigation was not designed to look at mortality rates, but the authors say the data it provided, when compiled with other estimates, suggest 0.02 percent of cases -- one in 5,000 -- ended in death.
That figure is in the low range of death tolls from annual "seasonal" flu.
By comparison, the 1918-1919 pandemic of "Spanish" flu, which killed tens of millions of people, is estimated by some to have had a case fatality ratio of 2.5 to three percent.
The study was led by the WHO and Imperial College London.
Data came from, among others, Britain, China, France, Germany, Hong Kong, India, Iran, the United States and Vietnam. The results from these countries, were then extrapolated for a planetary-wide figure.