A vaccine is a biological preparation that provides active acquired immunity against a particular disease. Vaccines contain weakened or killed microorganisms, their surface proteins, or their toxins that stimulate the body’s immune system to recognize the threat and destroy it.
When your immune system is stimulated by a vaccine, it remembers the pathogen and can rapidly attack it again in the future if you get exposed to it without having to actually experience the illness. Vaccines also prevent people from spreading the germ to others. That’s a key part of herd immunity, which helps everyone stay healthy.
There are many different types of vaccines. Some use a weakened (attenuated) live virus that has been carefully engineered so it can’t cause serious illness, but still has the ability to stimulate an immune response. This type of vaccine includes the measles, mumps, and rubella (MMR) vaccine and the varicella (chickenpox) vaccine. Others use a killed (inactivated) virus that has been chemically altered so it can’t reproduce or cause illness. This is the case for the polio, influenza, and hepatitis A vaccines, as well as the whooping cough (pertussis) vaccine.
All vaccines undergo extensive lab testing and clinical trials before they are licensed to be used in people. Vaccines are safe, and the benefits far outweigh any risks. CDC, the Food and Drug Administration, and other public health agencies routinely monitor vaccine safety and investigate any possible problems as soon as they occur.