Contraction of seasonal flu may offer protection against the H1N1 virus.

Allowing children to fall ill with seasonal flu may protect them against more dangerous pandemic strains, scientists have said.

Vaccinating children aged six months to five years against seasonal flu might not be in their best interests, according to a team of Dutch experts.

Flu jabs for healthy children are recommended in the US and some European countries, but not currently in the UK.

The three Dutch doctors, led by Dr Guus Rimmelzwaan, from Erasmus Medical Centre in Rotterdam, set out their view in an article published online by The Lancet Infectious Diseases.

They said research showed that infection with "seasonal" influenza A viruses could induce immunity against unrelated sub-strains. "This so-called heterosubtypic immunity does not provide full protection, but can limit virus replication and reduce morbidity (illness) and mortality of the host," the doctors wrote.

One new sub-strain of influenza A is the H1N1 virus responsible for swine flu. Another is avian or bird flu which, although very rare in humans, can be deadly.

The experts added: "Preventing infection with seasonal influenza viruses by vaccination might prevent the induction of heterosubtypic immunity to pandemic strains, which might be a disadvantage to immunologically naive people, such as infants."

The current swine flu pandemic could provide an opportunity to investigate the theory, they said. Comparing hospital admissions and death rates of vaccinated and non-vaccinated infants would show whether earlier seasonal flu infection could protect children against swine flu.

Writing in the same journal, two Finnish doctors, Terho Heikkinen and Ville Peltola, from Turku University Hospital, disagreed. They argued that the benefits of vaccinating young children against seasonal flu far outweighed the disadvantages.

Both teams agreed on the need for more effective flu vaccines which induced broader immune responses.