Most people infected with the flu virus show no symptoms, British researchers report today.
Fewer than one in four of those infected with the viruses fall ill, according to the new study.
The findings apply to both seasonal flu and the 2009 H1N1 swine flu pandemic, according to the researchers.
The findings come from a study of five groups of households in England over a six year period. Researchers took blood samples to identify people carrying the flu virus.
Researchers found that 18% of people were infected during the 2009 pandemic but 77% of these people suffered no symptoms, the researchers reported in The Lancet Respiratory Medicine.
Just 17% of those infected visited a doctor.
The findings may be seen to confirm fears that flu spreads through healthy people. The UK government has launched a programme of childhood vaccination to try to stem this spread.
Researcher Dr Andrew Hayward, from University College London, UK, said: "Despite its mild nature, the 2009 pandemic caused enormous international concern, expense, and disruption. We need to prepare for how to respond to both mild and severe pandemics.
"To do this we need more refined assessments of severity, including community studies to guide control measures early in the course of a pandemic and inform a proportionate response.”
Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study. Lancet Respir Med 17 March 2014 [abstract]
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