Combining certain child vaccines makes them less effective, a study shows.
The meningitis C vaccine is given routinely in the UK
An Oxford University team compared a combined meningitis C and pneumococcal jab to a single meningitis C vaccine.
They found the individual jab gave significantly better immunity against meningitis in a study of 240 babies, but both vaccines were considered safe.
The effectiveness of other vaccines for diseases including Hib and diphtheria were also compromised, the Journal of the American Medical Association said.
Lead researcher Jim Buttery said the findings suggested the combined vaccine, Pnc9-MenC, which is not yet licensed for use in the UK, "may not be a suitable replacement for individual vaccines".
He said the study also highlighted the unpredictability of the immune system and, once again, stressed the importance of assessing the effect of new vaccines on all other forms of immunisation.
The meningitis C vaccine is given to children during their first year in the UK and has helped reduce the disease by 87% since routine immunisation was brought in in 1999.
However, children are not routinely vaccinated against pneumococcal disease, which is the second biggest cause of bacterial meningitis and can also lead to blood poisoning and pneumonia, although the government's vaccine advisers are looking at whether to change that.
In the US, where an individual pneumococcal vaccine is given to children during the first 18 months, there has been a two thirds fall in the disease.
The researchers found 100% of the babies aged seven to 11 weeks given the individual jab produced good levels of antibodies, compared to 95% of those given the combined one.
Ann Coote, of the parents support group Jabs, said: "I believe other studies are also looking at this problem.
"It could be that other vaccine combinations are also not as effective and that is concerning for parents.
"There is a move towards more combinations, but this raises question about whether that is best for children."
And Linda Glennie, the Meningitis Research Foundation's head of research and medical information, said she was in favour of combination jabs as long as they were effective.
"The ideal would be if we could get to the point where there was a combined vaccination for all meningitis."
But she added while the researchers felt the immunity was compromised in this case, it was important to remember that just because a vaccine was not as effective as another did not necessarily mean it was below a safe immune level.
A Department of Health spokeswoman said the government's vaccine advisers were only looking at an individual vaccine.