The scare over possible side effects of the combined measles, mumps, and rubella (MMR) vaccine in the United Kingdom may be responsible for a recent increase in measles outbreaks there, according to new research. The authors of the study, published today in Science, fear that measles may reestablish itself in the United Kingdom if current vaccination rates persist.
Measles is a highly infectious viral disease that remains the leading cause of vaccine-preventable deaths in children. The MMR vaccine has successfully eliminated measles in 19 European countries and, possibly, the Americas. The vaccine was widely accepted in Britain until a study published in 1998 suggested a link between the MMR vaccine, autism, and a bowel disorder called Crohn's disease. Although subsequent research has refuted the claim, publicity surrounding the study sparked fears among parents, many of whom chose not to vaccinate their children with the MMR vaccine. By the end of 1998, only 88% of British 2-year-olds were being vaccinated, below the 95% recommended by the World Health Organization to eradicate measles.
The subsequent rise in the number of measles cases, which nearly doubled between 1998 and 1999 and remained elevated thereafter, appeared to be linked to the increasing pool of unvaccinated individuals in the population, says Vincent Jansen, a mathematical biologist at Royal Holloway University of London. Plotting the size and frequency of measles outbreaks in Britain between 1995 and 2002 against MMR vaccination rates, he and colleagues confirmed what the health experts had suspected: The drop in vaccinations coincided with a number of relatively large measles outbreaks. They also found that the "reproductive number," a measure of the transmissibility of the disease, was significantly higher from 1999 to 2002 than from 1995 to 1998, and it was approaching the level at which measles outbreaks would no longer fizzle out. If vaccination rates don't increase, the group says, measles could again become endemic in Britain.
The work is a "key first step" toward understanding how vaccination rates might affect future outbreaks, says Bryan Grenfell, a Cambridge University epidemiologist. But although the study establishes a link between vaccinations and measles outbreaks, more work is needed to sort out how other factors, such as the seasonality and spatial distribution of the disease, might affect the likelihood of future epidemics, he says. Mary Ramsay, a public health consultant at the Health Protection Agency and co-author of the study, says that as parents try to avoid the perceived risk associated with the MMR vaccine, they are putting their children at a very real risk of catching measles.