Scientists have said that they expect the swine flu virus to return for Britain during next winter’s flu season.

Tests have shown that in some regions, up to 85% of the population have not caught the flu strain, meaning that they have not developed immunity to it.

So far, between 10 and 30% of those offered the swine flu vaccine have had it done, meaning that a large number of people particularly at risk of developing complications from flu are still vulnerable.

There has been criticism of the government and of the World Health Organisation after large amounts of the vaccine were ordered, much of which is still unused after the pandemic proved less widespread than expected.

Professor Nial Ferguson, who is the Director of the MRC Centre for Outbreak Analysis and Modelling at the Imperial College department for Infectious Disease Epidemiology warned that a resurgence of cases of the H1N1 flu strain has been noticed in America, with older people particularly likely to be infected.

The team said that they believed that the government’s response to the pandemic was proportionate taking into account the information they had about the flu at the time and Prof. Ferguson said that the way information was collected on the virus meant that the number of related UK deaths was probably underestimated. So far 474 deaths have been attributed to the flu, far short of the early estimates, which put potential deaths at 65,000.

His colleague Prof. Openshaw of Imperial’s Centre for Respiratory Infection, added that it was hard to predict how the influenza strain would behave when it returned.