There has been a campaign to immunise children against meningitis C
Three-quarters of children vaccinated against meningitis C lose their protection against the disease by their early teens, research suggests.
The Oxford team which did the work says its findings fuel calls for a booster jab to be offered to adolescents.
The study of 250 children aged six to 12, presented to a European conference, looked at immunity seven years after the jab was given.
UK experts agreed a booster may be needed in the future.
The research was carried out by the Oxford Vaccine Group at Oxford University.
The group tested the children, who had all been vaccinated against meningitis C, for levels of antibodies against the bacteria in their bloodstream.
It was found that just 25% of the children had sufficiently high levels of the antibodies to give them protection against the disease.
The researchers say that British children are still protected against the potentially fatal bacteria at the moment, through the existence of herd immunity.
That means that vaccination has significantly reduced the level of meningitis in the population, and so even people who are not vaccinated are also protected.
But the researchers, led by Professor Andrew Pollard, told the European Society for Paediatric Infectious Diseases (ESPID) meeting in Nice, France, that if herd immunity starts to decline many children will be vulnerable.
'All time low'
Professor Pollard said: "This study is just the latest to show that the personal protection given by meningitis C vaccines in early childhood doesn't last forever and several countries have now responded to these findings by introducing teenage boosters, before protection fails in the population."
Falling immunity levels against meningitis C vaccination have been reported in Greece, the Netherlands and Spain.
Austria, Canada and Switzerland have already introduced booster jabs.
Dr Jamie Findlow, deputy head of the Health Protection Agency's Vaccine Evaluation Unit in Manchester, said: "By giving each teenager a booster dose of meningococcal vaccine as they are entering adolescence, we can ensure that they are protected when they most need it."
Professor Ray Borrow, head of the unit, said: "Parents should not be worried - at the moment cases of meningitis C are at an all time low.
"In 2008-2009 in England and Wales there were just 13 cases - and nine of these were in adults over 25 who may not have been vaccinated.
"We and other researchers are looking at how and when a booster could be introduced, but it doesn't have to come tomorrow."
He said herd immunity should last until around 2015.
Sue Davie, chief executive of the Meningitis Trust said the Oxford team's research raised "significant concerns".
"Vaccination is the only way to prevent meningitis and save lives. We support the use of safe and effective vaccines and encourage people to receive the vaccines that are currently available.
"If, as a result of this research, a booster programme is introduced, we would actively encourage the introduction of this."
A Department of Health spokeswoman said: "The number of cases of meningococcal C disease is currently very low.
"All new research on vaccines will be reviewed by the Joint Committee on Vaccination and Immunisation."