What is vaccine shedding?

Vaccines stimulate your body to produce immunity against a disease. Some use live viruses while others use inactive or killed viruses or bacteria. 

Live vaccines contain a weakened or attenuated form of a virus or bacteria. This is, in contrast, to “killed” or inactivated vaccines. It might sound frightening at first to realize that a vaccine contains a weakened virus or bacteria, but these are altered so that they cannot cause disease—at least in people with healthy immune systems, and the majority of people without a healthy immune system as well. If a child (or adult) has a suppressed immune system, live vaccines are not given.

Live vaccines are thought to better simulate natural infections and usually provide lifelong protection with one or two doses.3 Most inactivated vaccines, in contrast, require multiple primary doses and boosters (years later) to get the same type of immunity. In some types of live vaccines, a second dose is given because some people don’t respond to the first dose, but that is not considered to be a booster.

Parents sometimes have a concern about whether their healthy children should get live vaccines if they will be exposed to someone else who has a problem with their immune system, especially if they are in close contact with someone that has compromised immunity.

Fortunately, except for OPV and smallpox, which aren’t typically used anymore, children who live with someone who has an immunologic deficiency can and should get most vaccines in the routine childhood immunization schedule, such as MMR, Varivax, and the rotavirus vaccines. It would be extremely rare for someone to contract one of these viruses from someone who got the vaccine.

A much greater concern than live vaccine shedding of a weakened strain would be that the unvaccinated child might get a natural infection with measles or chickenpox and pass that on to a person with an immune system problem.

  • Vaccine shedding is the idea that someone who was recently vaccinated becomes contagious with the disease and therefore, can shed the virus to someone else.
  • Vaccines contain killed or weakened viruses that are too weak to cause illness, and therefore cannot cause the disease to be spread from one person to another.

Very Well Health: Understanding Live Vaccines and Vaccine Shedding
Journal of Allergy and Clinical Immunology: Recommendations for liv


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As parents, determining how best to protect our children can be overwhelming and confusing. We’re here to help.

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About I Vaccinate

I Vaccinate provides information and tools based on real medical science and research to help Michigan parents protect their kids. Support is provided by the Michigan Department of Health and Human Services and the Franny Strong Foundation.

You’ve got questions. That’s a good thing.

As parents, determining how best to protect our children can be overwhelming and confusing. We’re here to help.

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