What does a killed virus mean in a vaccine?
Ask the doctor
Q. Is "killed" virus the same as an inactive virus in vaccines?
A. Immunization means giving all or part of a germ, or a changed piece of the germ, to cause the body to make an immune response that is just like the one acquired after natural infection. This usually presents little or no risk to the person getting the vaccine.
The idea is that the person getting an immunization ends up with protection against a disease, usually against an infectious agent. Some vaccines must be given more than once over time to build or keep up the protection. The effectiveness of a vaccine is measured by how well it protects against the natural disease.
Killed or inactivated is the same thing when we talk about vaccines. Most vaccines are made up of the infectious germ itself, either "killed" (inactivated) or "live-attenuated" (weakened). A person's immune status, age, and overall health must be taken into account before deciding which type of vaccine is given.
Killed vaccines contain dead viruses or parts of a virus that have been completely disabled and can no longer cause disease. Any viral part is inactivated in some way from causing harm to the person who receives the vaccine. Our immune systems are still able to build an immune response against the dead virus, so anyone can respond well if they come in contact and become infected with that virus at any time. Examples of vaccines that most often use killed virus include the shots against polio, influenza, and hepatitis A.
Live-attenuated viral vaccines contain very weakened forms of the naturally occurring virus. These attenuated viruses can infect the body's cells, which builds a much stronger immune response against the virus than the killed (or inactivated) types do. Being vaccinated with a live-attenuated form of a virus may cause mild symptoms, but it helps protect against developing severe symptoms if a person is exposed to the natural virus.
Examples of vaccines with live-attenuated viruses are the MMR (measles, mumps, and rubella), the rotavirus vaccine given by mouth, and the influenza vaccine sprayed into the nose. Since the risk of side effects is higher for live attenuated vaccines, it is usually recommended that high-risk patients receive killed or inactivated vaccines.
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About the Author
Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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