The outcome is simple. A child who isn't vaccinated will be susceptible to measles, mumps and rubella. All three can have serious complications.
The main one is encephalitis, where the virus gets into the brain. The risk of encephalitis is about one per 1 000 cases for measles and for mumps and about one per 6 000 for rubella. Up to 50 percent of children who get encephalitis are left with permanent brain damage.
Measles kill about one in 5 000 children in developed countries and can also cause SSPE (subacute sclerosing pan-encephalitis) in about one in 14 000 cases.
SSPE progressively destroys nerve cells in the brain, leading to mental deterioration and death.
The other big risk comes later in life. Mumps and rubella both increase a woman's risk of miscarriages and rubella also causes birth defects, including blindness, deafness and autism.
Overall, the combined risk from measles, mumps and rubella (if a child were unlucky enough to catch all three) would be about one in 500 of encephalitis, one in 14 000 of SSPE and one in 5 000 of dying.
But what chance does your child have of catching these
diseases? Parents who choose not to vaccinate their own child while hoping that everybody else will may be making a big mistake.
There is a certain level of protection, called "herd
immunity", that has to be maintained to stop a disease
spreading.
For measles and rubella this is between 80 and 90 per cent. MMR vaccine coverage in Britain has now dropped to 88 percent, the critical level.
Already outbreaks of infection are occurring in pockets of unvaccinated children. Since November 1999, 103 cases of measles have been reported in a religious community in Salford, near Manchester.
Coverage in the area had dropped to around 80 percent. And Northern Ireland, where coverage is down to 76 percent, is suffering its worst outbreak in seven years.
There are risks associated with the MMR vaccine, as with any vaccine, but the risks from the diseases themselves are far greater.
Side effects are minimal
The results of several large studies suggest that MMR causes about four adverse reactions per 100 000 doses. The most common adverse effects are mild and include tenderness, slight fevers and rashes.
More serious consequences include seizures, allergic reactions and anaphylactic shock. There is a 1 in 3 000 risk of a child having a seizure, and a 1 in a million chance of anaphylactic shock.
However, all of the children in the studies recovered,
and no deaths have been reported.
But there is also still a risk of catching measles, mumps or rubella, as the MMR vaccine is only about 90 percent
effective.
Riskier in the short term
This is one reason why two doses are given and after both doses about 1 percent of children remain suceptible. So a parent who does not vaccinate their child, leaves these other children at risk too.
Separate vaccines against measles, mumps and rubella
ultimately give children the same level of protection as MMR and the same risks of adverse effects.
But it is riskier in the short term, because it takes longer and children are more likely to catch measles, mumps or rubella before they've had all three jabs.