The swine flu outbreak of winter 2009-2010 was much more widespread than was previously realised, research suggests.
Blood samples taken from Scottish adults in March last year at the end of the H1N1 flu season showed that almost half were carrying antibodies to the virus.
Most of the 44 per cent who tested positive had contracted swine flu, although some had acquired immunity from a previous bout of flu, or had been vaccinated.
The research, led by the University of Edinburgh, shows that many cases of swine flu went unreported. Only 100,000 people consulted their GP regarding flu, out of about two million who are believed to have contracted the virus.
People living in the most deprived areas were twice as likely to have contracted the virus. Scientists add that it is possible that many people who were vaccinated against the virus were already immune.
Almost 1600 adults from the east of Scotland and Glasgow, who are participants in the Generation Scotland Scottish Family Health Study voluntary health scheme, took part.
The research, carried out in collaboration with the University of Strathclyde, Health Protection Scotland and West of Scotland Specialist Virology Centre, was funded by the Chief Scientist Office and published in the journal PLoS One.
Professor Mark Woolhouse of the University of Edinburgh's Centre for Infectious Diseases, who led the study, said: "This flu spread very quickly. Fortunately most cases were mild but this also means that they weren't reported. Testing for antibodies to flu could be invaluable in tracking future pandemics and targeting vaccination to those groups who most need it."
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