Researchers assessed blood samples taken as part of the Health Protection Agency's monitoring programme for immunity to H1N1.
These showed that infection rates in London were 10 times higher than original estimates suggested, with 32% of under-15s and 20% of 20-24-year-olds having been infected.
Writing in The Lancet researchers said that this serological study showed the true extent of H1N1 infection in the initial wave of the pandemic in England in 2009.
Previous estimates of the number of first-wave cases were based on people presenting to GP surgeries with influenza-like illnesses. These missed those people with mild disease and those who did not consult a doctor.
The researchers found that rates of infection with 2009 pandemic H1N1 influenza in the first wave were greatest in children younger than 15 years, with an estimated 42% of schoolchildren aged 5-14 years being infected in high incidence regions.
‘Children have an important role in transmission of influenza and would be a key target group for vaccination both for their protection and for the protection of others in the herd,' the researchers said.