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Vaccination is a vital tool for preventing various infectious diseases. A vaccine contains components from a disease-causing organism (pathogen), like a virus or bacterium.

Encountering these components, called antigens, stimulates your immune system, teaching it how to recognize and respond to the pathogen. This provides protection against the pathogen, should you be exposed to it in the future.

You may have heard of something called vaccine shedding. This is where a vaccinated individual can release the components of a vaccine.

While vaccine shedding can happen with a few vaccine types, it’s not possible with many others. This includes the COVID-19 vaccines. Keep reading to discover more about vaccine shedding and when it actually occurs.

Vaccine shedding is when an individual releases, or sheds, the components of a vaccine either inside or outside of their body.

This can only happen with a certain type of vaccine called a live-attenuated vaccine. Some examples of live-attenuated vaccines that are commonly given in the United States include the:

About live-attenuated vaccines

Live-attenuated vaccines contain a weakened form of a pathogen. These types of vaccines need to replicate within the body in order to produce an immune response.

Due to their weakened nature, the pathogens in these vaccines don’t cause disease. The exception to this is in immunocompromised individuals, for which live-attenuated vaccinations aren’t typically recommended.

Because live-attenuated vaccines can replicate, the weakened pathogen can be shed. But it’s important to note that shedding doesn’t equate with transmission, in which the weakened pathogen is passed to another person.

Should these pathogens be passed to another individual, they’re highly unlikely to cause disease. In fact, the only live-attenuated vaccine associated with significant infections due to shedding is the oral polio vaccine, which is no longer in use in the United States.

In addition to live-attenuated vaccines, there are several other types of vaccines. Unlike live-attenuated vaccines, none of these vaccine types contain live pathogens. Because of this, they cannot be shed.

Inactivated vaccines

Inactivated vaccines contain a whole, killed version of a pathogen. Some examples of inactivated vaccines are the:

Subunit, recombinant, polysaccharide, or conjugate vaccines

In this diverse group of vaccines, only small pieces or fragments of a pathogen are present, as opposed to the entire pathogen. Examples of such vaccines include the:

mRNA vaccines

The mRNA in these vaccines instruct cells on how to make proteins associated with a pathogen in order to generate an immune response. The Pfizer-BioNTech and Moderna COVID-19 vaccines are mRNA vaccines.

Viral vector vaccines

Viral vector vaccines use a modified virus to deliver instructions on how to make proteins associated with a pathogen in order to produce an immune response. The COVID-19 vaccines produced by Johnson and Johnson and AstraZeneca use an adenovirus vector.

Toxoid vaccines

Toxoid vaccines are made up of an inactivated form of a toxin produced by some bacterial pathogens. In this case, an immune response is generated to the harmful toxin produced by the pathogen, rather than against the pathogen itself.

Two examples of toxoids are those for tetanus and diphtheria, which are found in the:

The only COVID-19 vaccines that are currently authorized for emergency use are mRNA vaccines and viral vector vaccines. While you may have seen posts on social media about COVID-19 vaccines shedding, this is a myth. COVID-19 vaccines cannot be shed.

This is because none of the COVID-19 vaccines contain live SARS-CoV-2, the virus that causes COVID-19. The only COVID-19 vaccines that are currently authorized for emergency use are mRNA vaccines and viral vector vaccines.

Let’s take a look at how each of these vaccine technologies works.

mRNA vaccines

mRNA vaccines contain genetic material called RNA. The mRNA is packaged within a protective shell called a lipid nanoparticle, which is essentially a tiny ball of fat. This allows the mRNA to effectively enter your cells.

The mRNA in the vaccine tells your cells how to make spike protein, a protein found on the surface of the novel coronavirus. Once the cells have produced the spike protein, the mRNA is broken down.

Viral vector vaccines

Viral vector vaccines use a modified adenovirus to deliver instructions on how to make spike protein.

In nature, adenoviruses can cause illnesses like the common cold. But the adenovirus used in the vaccine has been modified so that it can’t make more of itself (replicate) or cause illness.

Once inside the cell, the adenovirus releases the genetic material that tells the cell how to make spike protein. After this has happened, the adenovirus itself is broken down.

An analogy is to think of the adenovirus as a shipping container. It simply delivers its contents to the proper location before being disposed of.

What happens to the spike protein?

In both of these vaccine technologies, the spike protein that’s produced is transported to the surface of the cell. This allows it to be detected by the immune system.

Once your immune system recognizes the spike protein as foreign, it will work to generate an immune response against it. This immune response is targeted specifically at the spike protein.

As such, vaccine-generated spike proteins are eventually destroyed by your immune system. They can’t accumulate or circulate significantly in your body, nor can you shed them into the environment.

Some research has shown that very sensitive tests can detect minute levels of a piece of the spike protein in the blood in the days after vaccination. But these pieces of spike protein rapidly decline as the immune response kicks in.

It’s technically possible for any live-attenuated vaccine to shed. But in most instances, documented cases of this are rare.

The oral polio vaccine (OPV) is responsible for the most harmful infections related to vaccine shedding. The live-attenuated virus used in this vaccine can be shed from the body in feces.

In very rare cases, the virus used in the OPV can mutate and become harmful, potentially leading to paralysis. In countries that still use the OPV, this is estimated to occur in 2 to 4 out of every million live births each year.

Since the year 2000, the OPV is no longer licensed or available in the United States. Now, all polio vaccines given in the United States are inactivated vaccines.

Other live-attenuated vaccines for which shedding has been documented include the:

  • Flu nasal spray vaccine: Shedding of the virus used in this vaccine is common, particularly in younger individuals, according to the Centers for Disease Control and Prevention (CDC). While transmission of these viruses can occur, it’s rare and not typically associated with symptoms.
  • Chickenpox vaccine: According to the CDC, there have been reports of only 11 healthy vaccinated individuals worldwide spreading the chickenpox vaccine virus to 13 unvaccinated people.
  • Rotavirus vaccine: Rotavirus vaccine virus can be shed in feces for days after vaccination. An older 2011 study in twins found that the vaccine virus could be transmitted to unvaccinated individuals, but wasn’t associated with symptoms.
  • MMR vaccine: The rubella part of the MMR vaccine may be present in the breastmilk of recently vaccinated mothers. But transmission of the vaccine virus to breastfeeding infants is generally believed to be unlikely or rare.

Vaccine shedding is when components of a vaccine are released into the body or out into the environment. This can only happen in live-attenuated vaccines that contain a weakened form of a pathogen.

Other vaccine types cannot lead to vaccine shedding because they don’t contain live pathogens. This includes all of the currently available COVID-19 vaccines.

While live-attenuated vaccines can be shed, it’s unlikely that the weakened pathogens in these vaccines can be transmitted to unvaccinated individuals. When this does happen, it typically doesn’t result in symptoms.