Schoolchildren In Some areas Had Swine Flu

Estimates were based on the numbers of people seeing their GP or contacting the flu line with symptoms but this did not capture the cases which were mild or had no symptoms, the Health Protection Agency experts said.

In an article published online in The Lancet medical journal Prof Elizabeth Miller and colleagues described how blood samples taken before and after the swine flu outbreak has found many more people had the disease.

Before the outbreak only 1.8 per cent of children aged up to four years had any immunity against H1N1 rising to almost a third of people over 80 who were probably exposed to very similar viruses in previous pandemics.

But in August and September the proportion of children aged up to four with H1N1 antibodies rose to one fifth in London and the West Midlands. Almost half of children aged between five and 14 had antibodies, they said.

Prof Miller wrote: “Rates of infection with 2009 pandemic H1N1 influenza in the first wave were greatest in children younger than 15 years of age, with an estimated 45 per cent of schoolchildren aged five to 14 years being infected in high incidence regions.

"This finding is consistent with the high level of susceptibility in children and the increased potential for transmission that occurs within schools. We also showed substantial differences between regions in the extent of infection during the first wave.

“This serological study shows the true extent of H1N1 infection in the initial wave of the pandemic in England in 2009. Its findings should be applicable to other countries that have experienced a similar first wave.”

In a separate study it was found that because so many children were infected in the first wave that by the time the second wave hit, after the schools reopened in September, there was little impact the vaccine could make on the spread of the disease.

This supports the approach taken by government to vaccinate people at risk of complications if they contracted swine flu rather than a blanket programme aimed at slowing the spread of the disease.

Prof Miller added “This model, together with the serological data, would suggest that by the time vaccine became available in the UK in late October, 2009, the potential for mitigating the overall effect of the second wave by vaccination was limited.

“Around one child in every three was infected with 2009 pandemic H1N1 in the first wave of infection in regions with a high incidence, ten times more than estimated from clinical surveillance. Pre-existing antibody in older age groups protects against infection. 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 through herd immunity.”