MMR vaccine. Syringe and vial on a counter. This combined vaccine protects infants from three viral diseases: measles, mumps and rubella. The first injection is given when a child is two years old, followed by a booster three years later. The vaccine contains inactivated extracts of the three viruses. When injected, the vaccine stimulates the body's immune system to produce antibodies, but without causing infection. The immune system can then respond quickly against future infections by the viruses. In recent years, there has been public concern about a possible link between the MMR vaccine and autism. However, epidemiological studies suggest that there is no link.