MMR stands for measles, mumps and rubella, and is made from live, but
weakened forms of all three viruses. It was first licensed for use in the US in
1975. Europe followed later with Finland adopting it in 1982 and Britain in
1988.
There are six formulations on the market, each containing slightly different
combinations of weakened virus strains. Britain uses only the MMR vaccine
produced by Merck, the most commonly used vaccine throughout the world.
To develop and produce a new MMR vaccine, researchers first isolate samples
of each virus from infected patients. Then they grow these viruses in cell
cultures of human or animal cells. Scientists select weakened strains either by
testing them on lab animals, or more often by sequencing viral DNA to look for
mutations in the genes for virulence.
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Weakened or “attenuated” viruses are extracted from the cell culture fluid
and freeze-dried. The dried measles, mumps and rubella strains are mixed
together and the vaccine stays stable until added to distilled water for
injection. It has to be used within six hours. Each new strain is tested on
people before use, and every new triple strain combination must go through full
clinical trials.
Children under a year old are rarely vaccinated with MMR in developed
countries, because they are protected by antibodies from their mothers. Infants
in Britain receive their first MMR shot aged 15 to 24 months. In common with
many other countries, they also have a booster shot when they are three to five
years old.
