The MMR vaccine is a widely-used combined vaccine for measles, mumps and rubella. It is generally administered to children around the age of 1 year, with a booster dose before starting school (i.e. age 4/5)
Recently, controversy has arisen because some scientists claim to have found evidence that, in a very small fraction of cases, the vaccine may be linked to the development of autism. However, most scientific studies show that there is no link -- the incidence of autism in children who have been vaccinated is not significantly higher than that in those who have not been vaccinated.
The alarm was raised principally by a group lead by Dr. Andrew Wakefield which was published in the respected journal The Lancet. Many lurid media studies followed, leading to a widespread belief by the public that the story was proven beyond doubt. Rebuttals by UK Government/UK National Health Service (NHS) doctors and scientists were popularly disbelieved, partly because Government pronouncements on safety had been widely discredited in the 'Mad Cow' (Bovine Spongiform Encephalopathy, or BSE) affairs, and partly because it was believed that the NHS could not afford to pay for the separate vaccines. A sort of conspiracy theory type belief grew up which later research cannot entirely dispel. Thus, many parents are worried by the possibility, and would prefer to have the three vaccines administered separately. In the UK, a large part of the controversy is that only the combined vaccine is available on the UK National Health Service; those who do not wish to have it given to their children must either have the separate vaccines given privately, or not vaccinate their children at all. The Prime Minister, Tony Blair has refused to state whether his son, Leo Blair has received the vaccine.
Since the controversy began, there has been a noticeable increase in the incidence of measles in the UK, which has been attributed to the lower rates of vaccination. This drop in vaccination rates was one of the reasons why the UK Government was against single vaccines - people are less likely to go through three vaccinations than one single one. Other factors, such as the danger caused by rubella to pregnant mothers and their children were also posited as being relevant.
In the longer term, however, extensive studies of thousands of children in various parts of the world (studies which are more extensive and detailed than that initially done by Dr. Wakefield's team) have failed to show the link between autism and the MMR vaccine. Ironically, the publicity which surrounded the controversy in the UK will doubtless lead to more cases of the three diseases and birth defects occasioned by rubella both amongst the unvaccinated population and even the vaccinated population, as the UK populace are now at a level of vaccination where the protection offered by herd immunity fails.