A US study has re-ignited the debate about the safety of the MMR vaccine by suggesting it may raise the risk of neurological problems.
The MMR vaccine is highly controversial
Leading experts - who have repeatedly stressed the vaccine is safe - have expressed doubts about the validity of the study, claiming it is deeply flawed and that it appears in a journal not considered to be among the most prestigious in its field.
They are concerned that continuing suspicion about the safety of the vaccine - despite overwhelming scientific evidence to the contrary - will see vaccination rates dip to levels which make a measles epidemic more likely.
Last year there were 310 cases of measles in the UK - the highest number since MMR was introduced in 1988.
But UK campaigners, who want single jabs for measles, mumps and rubella to be made widely available, say the latest research justifies their continuing belief that MMR is not safe.
Because of serious methodological flaws, the conclusions concerning the association between MMR and DTP vaccine and the outcomes studied cannot be justified
The researchers compared the controversial vaccine with another vaccine given to children to protect them from diphtheria, tetanus and pertussis (DTP).
They found that the MMR vaccine was associated with a significant increase in the incidence of serious neurological disorders when compared to the other vaccine.
The researchers based their findings on an analysis of data collected by the US Vaccine Adverse Events Reporting System between 1994 and 2000.
During that time nearly 25 million children were given a first dose of MMR. Of these, 133 developed a neurological problem - either autism, mental retardation, a disorder called cerebellar ataxia or permanent brain damage.
The risk of autism was five times greater in those given MMR than it was in those given DTP.
The research was led by Dr Mark Geier, a geneticist at the National Institutes of Health.
He suggests that the MMR vaccination programme should continue - but with a version that contains dead, rather than live virus material to cut the risk of adverse reaction.
Proportion of two-year-olds in UK receiving MMR
April-June 1995 - 92.5%
April-June 1998 - 89.7%
April-June 2001 - 84.2%
Oct-Dec 2002 - 81.0%
Experts say a 95% vaccination rate is needed to protect all children
If the live vaccine continues to be used, than parents should be able to opt for separate single jabs, he says.
Writing in the journal International Pediatrics, he said: "It is clear that with the potentially globally destructive effects of natural measles, mumps and rubella infections that continued vaccination is necessary, but improvements in MMR vaccine is needed to improve its safety."
Dr Mary Ramsay, of the Health Protection Agency, described the research as "seriously flawed".
She said: "The authors have estimated the rate of reported adverse reactions following MMR (given at 15-18 months of age) and compared this to the estimated rates of the same conditions reported following DTP vaccination (given at 2, 4 and 6 months) of age.
Measles cases in England and Wales
1998 - 56
1999 - 94
2000 - 100
2001 - 72
2002 - 308
2003* - 160
* Until 16 May
"Regardless of the other weaknesses of the data, the failure to compare children of the same age is enough to explain the apparent excess of cases reported in the MMR group."
A Department of Health spokesman said previous research by the same authors, using similar methodology, had been carefully reviewed by the Committee on Safety of Medicines (CSM).
"The advice of the CSM was that this type of analysis cannot used to determine and compare the incidence of adverse reactions associated with different vaccinations.
"Therefore, because of serious methodological flaws, the conclusions of the authors concerning the association between MMR and DTP vaccine and the outcomes studied cannot be justified."
But Jackie Fletcher, of the campaigning group Jabs, called on the government to reassess its policy in light of the new study.
Currently, single dose vaccines for measles, mumps and rubella are not available on the NHS.
She said similar evidence presented to the Department of Health by Jabs had been dismissed as "anecdotal and insignificant".
"It is six years since the UK Government and vaccine policy makes were told of this problem and they have failed to investigate it properly by setting up a scientific, clinical examination of the damaged children."
Mrs Fletcher said ministers had also failed to improve the "inadequate" adverse events surveillance system, or to keep health professionals fully informed of the debate.
Dr Peter Mansfield, a GP who offers parents single jabs, said the new research was significant because it was the first time hard evidence about the problems associated with MMR had been shown in a big population study, rather than individual cases.