Key facts about the MMR vaccine and the diseases it is designed to prevent - measles, mumps and rubella.
What is measles?
Measles is an infection caused by a virus, normally acquired in childhood, most commonly in the one to four year age group.
It is highly infectious, and can be spread by coughs and sneezes, and is most infectious before the trademark rash appears.
What are the symptoms?
The first symptoms include runny nose, sore eyes, a cough and fever.
Around the fourth day of the illness, a rash - flat red or brown blotches - may appear, usually starting on the forehead and spreading downwards.
There may also be diarrhoea, vomiting and abdominal pain.
While this may be the full extent of the illness in many, resolving itself within two weeks of the first symptoms, other complications may arise.
These include a severe cough and breathing difficulties, ear infections, pneumonia and eye infections.
However, in a very small number of cases, inflammation of the brain (encephalitis) may follow. This is extremely dangerous, as 25% of those affected are left with brain damage.
The most severe complication of measles - occurring in only one in 100,000 cases - is a slowly-progressive brain infection which does not normally show until some time after the original infection, and eventually causes seizures and death.
What is mumps?
Mumps is caused by the mumps virus. Mumps is spread from person to person through direct contact with saliva, secretions from the respiratory tract, and urine of an infected person.
What are the symptoms?
Mumps does not usually cause serious long term problems, the acute symptoms, such as severe swelling of the salivary glands under the jaw bone, can be very uncomfortable.
Adults are more likely to have serious complications if they become infected than children.
About one in five adult males who are infected suffer from testicular inflammation, which in rare cases can lead to sterility.
The infection can also be linked to meningitis.
Exposure to the virus in the first weeks of pregnancy may increase the rate of spontaneous abortion.
What is rubella?
Rubella, also called German measles, is also caused by a virus that is spread from person to person when an infected person coughs or sneezes.
What are the symptoms?
Symptoms of rubella may include a rash, slight fever, aching joints, headaches, discomfort, runny nose and reddened eyes.
The rash first appears on the face and spreads from head to toe.
The lymph nodes just behind the ears and at the back of the neck may swell, causing soreness and pain.
Many people with rubella have few or no symptoms, and only about half of the people who have the disease get a rash.
However, if a pregnant woman gets rubella during the first three months of pregnancy, her baby is at risk of having serious birth defects or dying.
How can these diseases be prevented?
The Measles, Mumps and Rubella vaccine (MMR) can prevent measles infection in 90% of all immunised children.
The remaining 10% may still be able to catch the diseases because they have not responded to the vaccine, or because the MMR has not worked, perhaps because it is faulty.
A second dose raises this level of protection from measles to 99%.
It immunises those people for whom the first vaccination did not work, aims to catch those who missed out on the first dose and boosts immunity for those who did.
Incidence of a disease goes up if immunisation rates go down because vulnerable people are more likely to be exposed to the infection.
The first dose of the vaccine is normally given by a GP at between 12 and 15 months of age. The booster is given at between three and five years.
Why is the MMR vaccine controversial?
Some parents believe that their children have developed either an autistic spectrum disorder, or the bowel disease Crohn's following MMR vaccination.
There is some anecdotal evidence, presented by pressure groups, including cases in which healthy children fell ill with these conditions immediately or shortly after the administration of the vaccine.
The UK government commissioned a large survey involving thousands of children to try to find out whether immunised children were at a higher risk of developing autism or Crohn's. They found no link.
Many other research studies have come to the same conclusion.
In 2001, the World Health Organization issued a statement "strongly supporting the use of MMR vaccine on the grounds of its convincing record of safety and efficacy."
However, further research is underway to try to determine why the number of children diagnosed with autism has increased over the past 20 years.
Are there any alternatives to MMR?
Prior to the introduction of MMR in the mid 1980s, a single measles vaccine was widely used in the UK. Single vaccines were also available for mumps and rubella.
However, the government decided in August 1999 to ban the widespread importation of the measles vaccine into the UK.
It is concerned that this vaccine may not be as effective against measles as MMR, and also that gaps between different vaccinations may render children at higher risk of infection.
However, parents have strongly campaigned for the right to be able to choose this vaccine - and some have even taken their children abroad to be vaccinated using the single jab.
The jab is also available in this country if a parent can persuade a doctor to organise importation "for personal use", and administer it even though it is currently unlicensed.
What are the effects of some parents refusing to have their children immunised with MMR?
To prevent outbreaks of measles, a high proportion of the population have to receive immunisation against the disease.
The target in the UK is 95% of each age group, and currently, partly due to fears about MMR, 88% of children are being immunised.
However, in some parts of the country, particularly in the bigger cities, rates have fallen below the 80% mark.
This greatly increases the risk of severe outbreaks of measles, which could mean some children are permanently damaged.
Jo Yarwood, programme manager at Health Promotion England said: "The best evidence shows that MMR is the safest way to protect children against measles, mumps and rubella.
"Separating the MMR vaccine is not good medical practice.
"It would mean that large numbers of children are left unnecessarily at risk from three potentially serious diseases.
"In the '70s, the diphtheria, tetanus and
whooping cough vaccine was split - coverage collapsed to just 30% for
whooping cough and there were three major whooping cough epidemics. Many
children died as a result."
Why are single vaccines not available?
At present there are four licences for measles vaccines in the UK and one licence for a mumps vaccine.
However, the companies holding these licences are not marketing or manufacturing single measles or mumps vaccines that meet the specification in these licences.
Therefore all the imported single measles and mumps vaccines are unlicensed in the UK.
The importation of unlicensed medicines (not only vaccines) when a safe and effective licensed alternative (MMR) is available is restricted under the Medicines Act by the Medicines Control Agency.
This restriction applies equally to NHS and private sectors.
But didn't the vaccines used to be given separately - so why can't we just reintroduce this policy?
The three measles, mumps and rubella vaccines have never been split up and given separately in infancy in the UK.
Before the introduction of MMR in 1988, measles vaccine was given to infants from age one.
And until five years ago rubella vaccine was given to girls at age 10 to 13 years.
Separate mumps vaccine has never been routinely given in the UK.