Understanding Live Vaccines and Vaccine Shedding

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Vaccines stimulate your body to produce immunity against a disease. Some vaccines use live viruses or bacteria, while others use inactivated (killed) viruses or bacteria. For some diseases, both live and inactivated versions are available. Depending on the population group, one type may be chosen over another.

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Despite some limitations to their use, live vaccines are safe and effective and won't cause disease because the virus has been significantly weakened.

Even so, there are some who have expressed concerns that live vaccines can trigger viral shedding, the process by which cells of the body will release viral particles and, by doing so, increase the risk of infecting others.

The risk of viral shedding is one of the many reasons why anti-vaccination ("anti-vax") proponents claim that vaccinations are unsafe, especially live ones. Yet, despite the theoretical risk of infection from vaccine-induced viral shedding, there remains little evidence that such a phenomenon poses any genuine public health risks.

Live vs. Inactivated Vaccines

Live vaccines contain a weakened (a.k.a. attenuated) form of a virus or bacteria that, when introduced into the body, will stimulate an immune response in the form of antibodies. This is in contrast to inactivated (killed) vaccines in which the virus or bacteria is dead yet is still recognized by the immune system as harmful, triggering the same antibody response.

Live vaccines are thought to better simulate natural infections and usually provide lifelong protection with one or two doses. Most inactivated vaccines, by contrast, require multiple primary doses and booster shots years later to get the same level of immune protection. (The tetanus vaccine, which uses inactivated tetanus toxin, is one such example of this.)

Live attenuated vaccines have a long history of safety and efficacy but also have their limitations. People who are immunocompromised, for example, typically avoid live vaccines because their lack of immunity can make them ill if one is given.

Live attenuated vaccines currently licensed in the United States include:

Two live attenuated vaccines commonly used in the past—the smallpox vaccine and the oral polio vaccine (OPV)—are no longer used in the United States.

Vaccine Shedding

Vaccine shedding is a term commonly used by anti-vaxxers to describe the risk of infection due to vaccine-induced viral shedding. They assert that the use of vaccines, most especially live attenuated vaccines, can promote the spread of infection by causing cells to release viral particles due to the activation of the immune system.

While it is true that viral shedding is a normal mechanism that facilitates viral transmission—and one that can be induced by certain vaccines—there is little to no evidence that vaccines can induce the level of shedding needed for a vaccinated person to infect others.

Viral shedding in and of itself does not translate to an increased risk of transmission. It is only when the level of shed viruses is high that transmission can occur.

To date, the only vaccine that can potentially increase the risk of infection is the oral polio vaccine (OPV), and that is no longer used in the United States. Moreover, the highest concentration of shed viruses caused by OPV was found in stool, making that the primary route of infection.

There are few other documented cases of vaccine-induced viral transmission in the medical literature. Among the reasons for this:

  • Although inactivated vaccines can also cause viral shedding, the consensus is that the level of shedding is inadequate to facilitate infection.
  • The chickenpox vaccine is not known to cause shedding unless a rare vesicular rash develops after vaccination. The risk, however, is thought to be minimal, and the Centers for Disease Control and Prevention (CDC) reports only five suspected cases of transmission out of 55 million doses of the varicella vaccine.
  • The rotavirus vaccine also causes shedding in the stool, the transmission of which can be avoided with routine hygiene practices, such as good hand washing.
  • Transmission of influenza following the use of the FluMist vaccine has not been seen in studies involving people with HIV, children receiving chemotherapy, and immunocompromised people in healthcare settings.
  • The rubella part of the MMR or MMRV vaccine may cause viral shedding into breast milk, although transmission of rubella to a breastfed baby by this means is thought to be rare to unlikely.

Even so, viral shedding is a phenomenon that may pose risks to immunocompromised people who have not been vaccinated against the shed viral type. To this end, normal hygiene may be the best defense along with adherence to recommended vaccinations for adults and children.


Although live vaccines don't cause disease because they are made with weakened viruses and bacteria, there is always a concern that someone with a severely weakened immune system could get sick after getting one.

This is why live vaccines are typically avoided in organ transplant recipients, people receiving chemotherapy, or those with advanced HIV infection, among others.

The decision to use or avoid a live vaccine in people with weakened immune systems is based largely on the degree of immune suppression, weighing the benefits and risks on a case-by-case basis.

For example, it is now recommended that children with HIV receive the MMR, Varivax, and rotavirus vaccines depending on their immune status (as measured by the CD4 T-cell count).


The benefits of vaccination almost invariably outweigh the potential risks. With that said, there are several precautions to consider if you are scheduled to receive a live attenuated vaccine. Among them:

  • Multiple live attenuated vaccines can be given at the same time, but, if they aren't, you should wait at least four weeks before getting another live vaccine so that they don't inadvertently interfere with each other.
  • Children scheduled to receive a solid organ transplant should be updated on their live attenuated vaccines at least four weeks before the transplant.
  • Children receiving daily steroids for 14 days or more should delay getting live vaccines for at least three months following termination of treatment as the steroids can reduce the efficacy of the vaccine by blunting the immune response.
  • Yellow fever vaccine should be avoided if you are breastfeeding as there have been three cases of vaccine-associated encephalitis in breastfed babies whose mothers had been recently vaccinated.

A Word From Verywell

Most live attenuated vaccines pose little risk to the recipient or those who may be in contact with them after vaccination. What poses the most risk is avoiding vaccination and not getting immunized. It not only places you or your child at greater risk of infection but can lead to outbreaks of vaccine-preventable diseases thought eradicated.

The re-emergence of measles, a disease declared eliminated in the United States back in 2000, is one such example of the consequences of vaccine avoidance.

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