MMR vaccine being drawn into a syringe. This combined vaccine protects infants from three viral diseases: measles, mumps and rubella. The first injection is given in a child's second year, followed by a booster three years later. The vaccine consists of weakened samples 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.