Lead author Dr. Andrew Hayward of the University College London said the Flu Watch study tracked five successive groups of households across England over six influenza seasons from 2006 to 2011.
"Reported cases of influenza represent the tip of a large clinical and sub-clinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention," Hayward said in a statement.
"Most people don't go to the doctor when they have flu. Even when they do consult they are often not recognized as having influenza. Surveillance based on patients who consult greatly underestimates the number of community cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die. Information on the community burden is therefore critical to inform future control and prevention programs."
Study participants provided blood samples before and after each influenza season, and all participating households were contacted weekly to identify any cases of cough, cold, sore throat or "influenza-like illness." Any person reporting symptoms was asked to submit a nasal swab on day 2 of illness to test for a variety of respiratory viruses.
The study, published in The Lancet, found on average 18 percent of the unvaccinated community were infected with influenza each winter season -- 19 percent during pre-pandemic seasons -- and 18 percent during the 2009 pandemic.
However, 77 percent of these infections showed no symptoms, and only around 17 percent of people with confirmed influenza visited their doctor, the study said.
The study indicates that primary care surveillance greatly underestimates the extent of infection and illness in the community, the researchers added.
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