Understanding Live Vaccines and Vaccine Shedding

Vaccines stimulate your body to produce immunity against an infection. Those known as live attenuated vaccines use a very weak form of a virus (occasionally, bacteria) to achieve this.

Using them means that a modified form of the threat, which the vaccine is meant to target, does enter the body. This is done to spur an immune system response, typically without causing illness.

Some people oppose live vaccine use because they think it can cause a virus to spread through a process called shedding. This article explains what that means and why there is little evidence to support this view.

Live Vaccines and Viral Shedding

When a live vaccine is used to build immunity against a specific virus or other pathogen, the body's response is "virtually identical" to what you would see if you were naturally infected.

Live vaccines have saved lives. The process works against many threats, from chickenpox to cholera. But some people say that live vaccines can cause viral shedding.

Shedding is what happens when cells in a person's body release viral particles. For example, the virus can go into the air. This may increase the risk of spreading the infection to others.

The risk of viral shedding is a top reason why some people hold anti-vaccination views. They say that the use of vaccines, especially live ones, is unsafe.

Live vs. Inactivated Vaccines

Live attenuated vaccines contain a weakened, mild form of a virus or bacteria. The vaccine is meant to stimulate an immune response in the form of antibodies, a type of protein that fights off infection.

The live vaccines stand in contrast to vaccines that use dead bacteria or viruses. These inactivated, or killed, vaccines are still recognized by the immune system as harmful. So they lead to the same antibody response.

Live vaccines are meant to simulate a natural infection. They are thought to be better in doing so than the killed vaccines. Usually, they provide lifelong protection with one or two doses. 

Most killed vaccines need to be given with more than one dose for a complete vaccination. People also tend to need boosters years later to keep the same level of immune protection. Your tetanus vaccine, which uses inactivated tetanus toxin, is one common example.

Live attenuated vaccines currently licensed in the United States include:

• Adenovirus vaccine (Adenovirus type 4 and type 7, for military use only)
• Chickenpox (varicella) vaccine (Varivax)
• Cholera vaccine (Vaxchora)
• Influenza nasal spray vaccine (FluMist)
• Measles, mumps, and rubella (MMR) vaccines (M-M-R II, Priorix)
• Measles, mump, rubella, and varicella (MMRV) vaccine (ProQuad)
• Oral typhoid vaccine (Vivotif)
• Rotavirus vaccines (Rotarix and RotaTeq)
• Smallpox vaccine (ACAM2000)
• Yellow fever vaccine (YF-Vax)

Two live attenuated vaccines commonly used in the past—a previous smallpox vaccine and the oral polio vaccine (OPV)—are no longer used in the U.S.

Bacille Calmette-Guérin (BCG) for tuberculosis is a live vaccine rarely used in the U.S. BCG and Vivotif are used to prevent a bacterial infection. All others are used to prevent viral infections. The BCG vaccine is also sometimes used as a treatment for bladder cancer.

Vaccine Shedding

When the term vaccine shedding is used by those who hold anti-vaccination views, they usually are referring to the risk of infection due to viral shedding. They say the shedding is caused by a vaccine that puts the virus itself into public circulation. Thus, the use of vaccines—especially live attenuated vaccines—would promote the spread of infection.

It is true that viral shedding is one way of spreading a virus. Certain vaccines can, in fact, lead to such shedding. However, there is little evidence that viral or bacterial vaccines can lead to the level of shedding that would cause a vaccinated person to transmit the infection to others.

To date, the only vaccine with the potential to raise the risk of infection is the oral polio vaccine (OPV). It is no longer used in the U.S.

Moreover, the viral shedding from OPV was concentrated in the stool (feces). This makes any contact with them, such as the fecal-oral route common with poor hygiene and hand washing, the primary way to cause such an infection.

There are few other documented cases of a virus that's spread because of a vaccine. These are some highlights and takeaways from medical research:

• Killed vaccines can also cause viral shedding, but most studies find the level of shedding is not enough to cause an infection.
• The chickenpox vaccine is not known to cause shedding unless it causes a rare vesicular rash in the body. The risk of transmission is thought to be very low. The CDC reports only five suspected cases out of 55 million doses of the varicella vaccine.
• Like OPV, the rotavirus vaccine causes shedding in the stool. Viral spread can be avoided with better hygiene, such as good hand washing.
• The rubella part of the MMR or MMRV vaccine may cause viral shedding into breast milk. This type of rubella spread to a breastfed baby is thought to be rare, if not unlikely.

Even so, viral shedding may pose risks to immunocompromised people when they have not been vaccinated against that specific virus. To this end, good hygiene may be the best defense. So is the routine practice of getting all the recommended vaccinations for adults and children.

Contraindications

Live vaccines don't cause disease. But they are made with weak forms of a virus or bacteria, and this means there is the risk that a person with a severely weakened immune system could get sick after getting one.

This is why organ transplant recipients typically avoid live vaccines. Among others, this also is true for people who have chemotherapy treatments and for those living with an advanced stage of HIV.

For example, it is now recommended that children living with HIV receive the MMR, Varivax, and rotavirus vaccines. But this depends on their immune status, measured by the CD4 T-cell count.

Precautions

Vaccines offer benefits that almost always 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:

• More than one live attenuated vaccine can be given at the same time. If they aren't, you should wait at least four weeks before getting another live vaccine. This will limit any interference between them.
• Children scheduled for a kidney or other solid organ transplant should have any live attenuated vaccines given at least four weeks before the surgery.
• Children who take high-dose corticosteroid drugs like prednisone for 14 days or more may need to hold off on any live vaccines until their treatment is ended. Parents should advise their healthcare provider if a child is taking these drugs and a live vaccine is recommended.
• Yellow fever vaccine should be avoided if you are breastfeeding. There have been rare cases of vaccine-associated encephalitis, or brain infection, in breastfed babies after a nursing parent was recently vaccinated.

Summary

A live attenuated vaccine contains a weak form of the live virus that it's meant to protect against. This is to spur the body to mount its own immune response in defense. Both live vaccines and inactivated, or killed, vaccines work this way, but the live vaccines may be better because they behave more like a natural infection.

Some people oppose the use of live vaccines. They base part of their anti-vaccination stance on vaccine shedding, or the belief that the vaccines will actually spread the virus rather than control it. This is because the weak, live virus enters the body through the vaccine. Some of those who oppose vaccinations say the virus can then be shed and passed to others.

This may be true in theory but not in practice. Live vaccines don't lead to shedding that causes high enough levels of the virus for it to spread, and therefore can't really cause other infections.

A Word From Verywell

When it comes to approved vaccines, the greater health risk likely comes from not getting your recommended vaccinations at all. The return of measles, once eliminated in the U.S., makes the risk clear. Avoiding a vaccine can place you or your child at greater risk of infection and lead to outbreaks of vaccine-preventable diseases.

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