Measles is on the verge of becoming endemic in Britain because of the reluctance of some parents to inoculate their children with the three-in-one MMR vaccine, researchers say.
Scientists have predicted that outbreaks of the viral disease will get worse as the proportion of unvaccinated children in the population increases. And the new research warns that if the number of unprotected children continues to grow at the present rate, a national epidemic is inevitable.
The study by the University of London and the Government's Health Protection Agency found that the spreading ability of measles had significantly increasedin the past five years, a period in which the MMR vaccine was said to be linked to autism. It estimates that the reproductive number a statistical term to describe how many people on average become infected from a single measles case almost doubled between 1995 and 2002.
Vincent Jansen from Royal Holloway College, lead author of the study published in the journal Science, said: "We are now close to the point where new infections will no longer fizzle out on their own. We are approaching the danger zone where measles could once again become an endemic disease in the UK. We are not there yet, but it may be going that way.
"We are reporting a correlation between the drop in vaccinations and the increasing size of measles outbreaks. The coincidence is suggestive of a causative connection, but we cannot draw this conclusion from our data. My hope is that this is a warning signal."
The research team investigated measles outbreaks involving more than one child going back to 1995 and found that they increased in frequency and size in late 1998, soon after the supposed link with autism and the three-in-one measles, mumps and rubella vaccine received publicity.
They estimated the reproductive number and found that it increased from 0.47 in the period from 1995 to 1998 to 0.82 in 1999-2002. When the reproductive number is smaller than one, an outbreak will fizzle out, but if it becomes greater than one then the infection spreads because, on average, each individual with measles passes it on to more than one other person.
"The increase in the reproductive number occurred almost immediately after the decrease in the MMR vaccine uptake," the researchers report.
"If the current low level of MMR vaccine uptake persists in the UK population, the increasing number of unvaccinated individuals will lead to an increase in the reproductive number and possibly the re-establishment of endemic measles and accompanying mortality," they say.
Mary Ramsay, a researcher at the Health Protection Agency, said that in some parts of the UK notably the South-east uptake of the MMR vaccine was about 70 per cent. If this was to occur nationwide, endemic measles could occur.
"There's no doubt there's a possibility if vaccine cover does not improve. I don't think we'll see a national epidemic this winter but we are likely to see bigger and bigger outbreaks in different parts of the country," Dr Ramsay said.
"I don't want to scaremonger. Parents should look at the evidence and the evidence is pretty clear that the MMR vaccine is not linked with autism," she added.
Although vaccine uptake is low among children less than two years old, it is higher among school-age children. "For this reason I feel confident that we're not going to see large epidemics sweeping through schools as they used to," Dr Ramsay said.