Researchers have said that around 1 in 5 of the population were infected in both recent outbreaks of seasonal flu and the 2009 H1N1 influenza pandemic, but just 23 per cent of these infections caused symptoms.

The Flu Watch study tracked five successive cohorts of households across England over six influenza seasons between 2006 and 2011. The researchers calculated nationally representative estimates of the incidence of influenza infection, the proportion of infections that were symptomatic, and the proportion of symptomatic infections that led to medical attention.

Participants provided blood samples before and after each season for influenza serology, and all participating households were contacted weekly to identify any cases of cough, cold, sore throat or 'flu-like illness'. Any person reporting such symptoms was asked to submit a nasal swab on day 2 of illness to test for a variety of respiratory viruses using Real-Time, Polymerase Chain Reaction (RT-PCR) technology.

The results show that on average 18per cent of the unvaccinated community were infected with influenza each winter season-19per cent during prepandemic seasons and 18per cent during the 2009 pandemic. But most (77per cent) of these infections showed no symptoms, and only around 17per cent of people with PCR-confirmed influenza visited their doctor. Compared with some seasonal flu strains, the 2009 pandemic strain caused substantially milder symptoms.

The study has been published in The Lancet Respiratory Medicine journal.