Scientists have long disputed whether swelling of the brain, or encephalitis, can occur in children as a complication of flu.

But now a team of scientists in Japan has found evidence of a direct link between infection with two strains of the influenza virus and the development of the life-threatening brain disorder.

Dr Shinji Fujimoto, of the Nagoya City University in Japan, led the research, which concluded that there was a compelling case in favour of flu vaccination for children under five years old.

Dr Fujimoto and his colleagues published their findings in The Lancet.

Children at risk

They said: "Our patients were very young, and such children with their first influenza infection so early in life may be at greater risk of influenza-associated encephalitis."

Jabs may protect children

While the occurrence of brain inflammation as a result of flu was rare, vaccination against influenza could prevent it, they concluded.

Currently the flu vaccine is only recommended for people aged over 75, people with long-term diseases such as asthma or diabetes which make them more prone to infection, and those who live in long stay residential accommodation where the virus can spread rapidly.

The study looked at 10 children aged between 22 months and four years old who had been hospitalised with brain disorders.

Screening technique

Researchers took samples of blood and spinal fluid from the children and examined them for evidence of recent flu infection.

They established the link by using a technique called polymerase chain reaction (PCR), which replicates samples of genetic material many times until there is enough of it to study.

Seven of the 10 children were found to have been recently infected with flu. Six of those had shown signs of brain inflammation within two days of contracting the virus.

Influenza-associated encephalitis is a recently-identified disorder that mainly affects infants and very young children in Japan and Taiwan.

To date there have been very few cases of the disorder affecting western children.

A Department of Health spokeswoman said that there were no plans to extend the health service programme of vaccination to include young children.

But she added: "The joint committee on vaccination and immunisation keeps on doing research to see whether there are implications for our vaccination policy."