What is the difference between live and killed vaccines?

Vaccines in the United States are classified as "inactivated" or "live attenuated."

An inactivated vaccine works when the immune system responds to a piece of a bacteria or virus or to a toxin produced by the germ.

Live “attenuated” vaccines mean they have been changed such that they do not cause disease. They grow inside the vaccinated person until the immune system recognizes the bacteria or virus and has the appropriate response, similar to a natural infection.
The difference between live and killed vaccines has to do with how they are made. Live, attenuated vaccines contain a living, although significantly weakened, version of a virus or bacteria. Measles, mumps and chicken pox vaccines are made with live viruses.

The benefit of a live vaccine is that a single dose often provides lifelong immunity. The downside is that because viruses and other pathogens naturally mutate, or change, the virus within the vaccine could also change, creating a stronger version of itself that the immune system would have difficulty combating. This was an issue with the early oral polio vaccines, but is generally not a problem with current live vaccines, which are much safer than the virus they protect against.

Only people with a suppressed immune system (such as those who have HIV, who are taking immunosuppressant drugs or who are being treated for cancer) should be concerned about receiving live vaccines because they could, conceivably, become infected with the virus. These vaccines also usually require refrigeration.

Inactivated vaccines contain a killed version of the pathogen. They are more stable (meaning they don't need refrigeration) and safer than attenuated viruses, but they don't cause as strong an immune response. Thus, the immunity they provide may not last as long and you might need a "booster" vaccine down the road.

Continue Learning about Vaccines & Immunizations

Vaccines & Immunizations

Vaccines & Immunizations

Vaccines are commonly given to children in the form of a shot to help prevent serious diseases like measles and mumps. Vaccines are developed using either dead strains of a disease, weakened strains, or strains of a different dise...

ase. As adults, we receive flu vaccines or may need a booster of childhood vaccines to retain immunity. Travelers may receive vaccines either as a condition of entry to a country, or on recommendation of health officials. Generally there is little or no reaction to a vaccine, but in some cases the vaccine may cause an allergic reaction or a temporary, mild illness. Some vaccines are not safe for pregnant women, so it’s important to check with a healthcare professional.

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.