What is Vaccine Shedding?
A vaccine is comprised of components from pathogens like a bacterium or virus.(1,2) When the body encounters these components, known as antigens, it stimulates your immune system, thus teaching it to recognize and respond to that particular pathogen. This gives protection to the vaccinated person against the particular pathogen, should the person get exposed to it again in the future.(3,4,5)
Vaccine shedding happens when a vaccinated person releases or sheds some components of the vaccine. This can happen either inside or outside of the body.
However, vaccine shedding happens only with a specific type of vaccine known as a live-attenuated vaccine. Some of the common examples of live-attenuated vaccines that are usually administered in the United States include:
- Flu nasal spray vaccine (FluMist)
- Measles, mumps and rubella (MMR) vaccine
- Rotavirus vaccine
- Chickenpox vaccine
Live-attenuated vaccines contain a weakened form of the pathogen, and these types of vaccines have to replicate inside the body in order to give an immune response. But, due to the weakened nature of the pathogens contained in the vaccine, the pathogens in these vaccines do not cause any disease. The exception to this is in immunocompromised people, which is why live-attenuated vaccines are usually not recommended for such individuals.(6,7)
Since live-attenuated vaccines can replicate, it is possible for the weakened pathogen to shed. However, it is essential to know that vaccine shedding is not the same as transmission. In transmission, the weakened pathogen gets passed to another person. Nevertheless, even if these weakened pathogens get transmitted to another person, they are not likely to cause any disease. The only live-attenuated vaccine linked with significant infections due to shedding is the oral polio vaccine, which is no longer in use in many countries around the world.(8)
Do COVID-19 Vaccines Shed?
The only vaccines approved for emergency use in COVID-19 are all viral vector vaccines and mRNA vaccines.
Even though you might have seen certain posts on social media about the COVID-19 vaccines shedding, but it is just a myth. COVID-19 vaccines do not shed.(9)
The reason why COVID-19 vaccines do not shed is that none of them contain live SARS-CoV-2, which is the virus that causes COVID-19.
Here’s an explanation of how mRNA and viral vector vaccines work.
How Does mRNA Vaccine Work?
mRNA vaccines are made from genetic material known as RNA. The mRNA gets packaged inside a protective shell known as a lipid nanoparticle, which is primarily just a tiny ball of fat. This lets the mRNA enter into the cells effectively.
The mRNA present in the vaccine lets the cells know how to make the spike protein, which is a protein found on the surface of the novel coronavirus. After the cells have manufactured the spike protein, the mRNA is then broken down.(10)
How Does Viral Vector Vaccine Work?
Viral vector vaccines function with an altered adenovirus to deliver instructions to the cells on how to manufacture the spike protein.(11)
In nature, there are adenoviruses that can cause diseases like the common cold. However, the adenovirus that is used in the vaccine gets modified so that it is not able to replicate and make more of itself or cause illness. Once the adenovirus enters the cell, it releases the genetic material that lets the cell know how to make the spike protein. After this, the adenovirus gets broken down.
The adenovirus can be thought of as being a shipping container that only delivers the contents to the correct location and then gets disposed of.
What is the Role of the Spike Protein?
In either of the vaccine technologies, the spike protein that gets produced is transported to the cell’s surface, which allows your immune system to detect it. Once the immune system recognizes that the spike protein is a foreign invader, it begins working to generate an appropriate immune response against it. This immune response is targeted specifically at the spike protein.(12)
Vaccine-generated spike proteins are eventually destroyed by the immune system. They cannot be allowed to accumulate or circulate in significant amounts in the body, and neither can you shed these spike proteins into the environment.
Some research has indicated that there are some very sensitive tests that can detect even minute levels of a piece of spike protein leftover in the blood in the days following vaccination. However, these pieces of spike protein start to rapidly decline as the immune response begins.(13)
Which Vaccines Do Shed?
Technically, it is possible for any live-attenuated vaccines to shed, but documented cases of this are very rare.
The oral polio vaccine (OPV) is known to cause some of the most harmful infections related to vaccine shedding. When the live-attenuated virus is used in the OPV, the vaccine can be shed from the body through the feces. In some very rare cases, the virus used in the OPV can mutate and cause harm, even leading to paralysis. In countries that still use OPV, this is believed to occur in two to four out of every million live births each year.(14) Since 2000, the OPV vaccine is no longer available or even licensed in the United States. All polio vaccines administered in the US are inactivated vaccines.
There are some other live-attenuated vaccines for which shedding has been documented. These include:
- Chickenpox Vaccine: According to data from the US Centers for Disease Control and Prevention (CDC), there are reports of only 11 healthy vaccinated people in the world spreading the chickenpox vaccine to 13 unvaccinated people.(15)
- Flu Nasal Spray Vaccine: Shedding of the flu virus in this vaccine is actually quite common, especially in younger individuals. According to the CDC, while the transmission of these flu viruses can occur, it is still rare and not usually associated with any symptoms.(16)
- MMR Vaccine: The rubella part of the MMR vaccine has been found to be present in the breastmilk of mothers who recently got the vaccine. However, the transmission of the vaccine virus to breastfeeding children is believed to be rare or even unlikely.(17)
- Rotavirus Vaccine: The rotavirus vaccine virus is believed to be shed through feces for several days after getting the vaccine. A 2011 study done on twins found that the vaccine virus was getting transmitted to unvaccinated people, but it did not cause any symptoms.(18)
Any Precautions To Take Against Vaccine Shedding?
Vaccine shedding is highly unlikely to cause any harm to most healthy people. However, it can pose a risk to people who are immunocompromised, including people like:
- Those who are taking immunosuppressive medications
- People taking treatment for cancer
- People living with the human immunodeficiency virus (HIV)
- Recipients of bone marrow or an organ transplant
If you have recently taken a live-attenuated vaccine, it is a good idea to avoid coming in close contact with immunocompromised people for a few days after your vaccination. Additionally, frequent hand washing during this period can also help.
Vaccine shedding happens when specific components of the vaccine get released into the body or out into the environment. This usually only happens in the case of live-attenuated vaccines that contain a form of the weakened pathogen. Other types of vaccines are not known to cause vaccine shedding since they do not contain any live pathogens. This includes all of the COVID-19 vaccines currently available.
Even though live-attenuated vaccines can be shed, it is unlikely that the weakened pathogens being shed can be transmitted to individuals who are not vaccinated. In the rare cases that this does happen, it usually does not result in any symptoms.
- Senne, D.A., King, D.J. and Kapczynskiº, D.R., 2004. by Vaccination. Dev Biol. Basel, 119, pp.165-170.
- Pulendran, B. and Ahmed, R., 2011. Immunological mechanisms of vaccination. Nature immunology, 12(6), pp.509-517.
- Iwasaki, A. and Omer, S.B., 2020. Why and how vaccines work. Cell, 183(2), pp.290-295.
- Siegrist, C.A. and Lambert, P.H., 2016. How vaccines work. In The vaccine book (pp. 33-42). Academic Press.
- Cohen, J., 2021. Can immune responses predict which vaccines work best?.
- Minor, P.D., 2015. Live attenuated vaccines: Historical successes and current challenges. Virology, 479, pp.379-392.
- Ohmit, S.E., Victor, J.C., Rotthoff, J.R., Teich, E.R., Truscon, R.K., Baum, L.L., Rangarajan, B., Newton, D.W., Boulton, M.L. and Monto, A.S., 2006. Prevention of antigenically drifted influenza by inactivated and live attenuated vaccines. New England Journal of Medicine, 355(24), pp.2513-2522.
- Block, S.L., Yogev, R., Hayden, F.G., Ambrose, C.S., Zeng, W. and Walker, R.E., 2008. Shedding and immunogenicity of live attenuated influenza vaccine virus in subjects 5–49 years of age. Vaccine, 26(38), pp.4940-4946.
- Lipsitch, M. and Dean, N.E., 2020. Understanding COVID-19 vaccine efficacy. Science, 370(6518), pp.763-765.
- Jan, E., Mohr, I. and Walsh, D., 2016. A cap-to-tail guide to mRNA translation strategies in virus-infected cells. Annual review of virology, 3, pp.283-307.
- Ura, T., Okuda, K. and Shimada, M., 2014. Developments in viral vector-based vaccines. Vaccines, 2(3), pp.624-641.
- Xia, X., 2021. Domains and functions of spike protein in sars-cov-2 in the context of vaccine design. Viruses, 13(1), p.109.
- Ogata, A.F., Cheng, C.A., Desjardins, M., Senussi, Y., Sherman, A.C., Powell, M., Novack, L., Von, S., Li, X., Baden, L.R. and Walt, D.R., 2021. Circulating SARS-CoV-2 Vaccine Antigen Detected in the Plasma of mRNA-1273 Vaccine Recipients. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America.
- Altamirano, J., Sarnquist, C., Behl, R., García-García, L., Ferreyra-Reyes, L., Leary, S. and Maldonado, Y., 2018. OPV Vaccination and Shedding Patterns in Mexican and US Children. Clinical Infectious Diseases, 67(suppl_1), pp.S85-S89.
- Cdc.gov. 2021. Safety Information for Chickenpox (Varicella) Vaccines | Vaccine Safety | CDC. [online] Available at: <https://www.cdc.gov/vaccinesafety/vaccines/varicella-vaccine.html> [Accessed 14 October 2021].
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- Vaccine, M., 2021. Measles-Mumps-Rubella-Varicella Vaccine. [online] Ncbi.nlm.nih.gov. Available at: <https://www.ncbi.nlm.nih.gov/books/NBK501687/> [Accessed 14 October 2021].
- Rivera, L., Peña, L.M., Stainier, I., Gillard, P., Cheuvart, B., Smolenov, I., Ortega-Barria, E. and Han, H.H., 2011. Horizontal transmission of a human rotavirus vaccine strain—a randomized, placebo-controlled study in twins. Vaccine, 29(51), pp.9508-9513.
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