Vaccine shedding

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Vaccine-induced viral shedding (erroneously termed vaccine shedding by anti-vaccine misinformation) is ordinary viral shedding with the distinction being that it followed administration of an attenuated vaccine (also known as a “live-virus vaccine”), which is a specific vaccine technology that uses an attenuated form of a live virus. The overwhelming majority of vaccines, however, are not attenuated (live virus) vaccines, and therefore cannot cause vaccine-induced viral shedding.

The specific use of the erroneous and ambiguous term “vaccine shedding” has recently come into general parlance of anti-vaccine activists linked to misinformation relating to COVID-19, even though none of the many COVID-19 vaccines developed around the world use live-virus vaccine technology. Despite this, a COVID-19 “vaccine shedding” conspiracy theory has consequently emerged as one of many items of COVID-19 misinformation pushed by COVID-19 conspiracy theorists leading to vaccine hesitancy among some people.[1][2][3] With the exception of the oral polio vaccine (OPV), which was discontinued in the USA in 2000,[4] there have been few documented cases of vaccine-induced viral shedding that has infected contacts of a person vaccinated with an attenuated (live-virus) vaccine.[5]

Vaccine-induced viral shedding is no different to ordinary viral shedding during a regular infection with a virus, which is part of the normal mechanism of virus transmission.[6]

Shedding is only possible, but even so, unlikely, with an attenuated vaccine (live-virus vaccine), and it is impossible with other vaccine technologies such as inactivated vaccine (killed-virus vaccines), viral vector vaccine, RNA vaccines,[7] or subunit vaccines (a vaccine technology using only isolated proteins of a virus).

Only a small number of vaccines use technology that contain live virus which can theoretically infect others. Nevertheless, not all pathogens are shed; shedding does not equal transmission; and transmission does not always cause disease.[8]


The only human vaccine to have caused any significant number of infections is the oral polio vaccine (OPV),[4][9] and this vaccine is no longer used in developed countries.[4] However the OPV is still used in Asia and Africa, leading a small number of vaccine-derived polio infections each year.[10] In 2016, the formulation of the OPV was changed to remove the already extinct and most problematic type 2 polio strain from the OPV.[10] The route of infection is through contact with faeces, and some live vaccines, like the viruses they prevent, are shed in stool for up to 28 days.[11] Normal hygiene is sufficient to prevent infection[12] but immunocompromised individuals need to be especially diligent.[13]

Other attenuated vaccines show no significant viral shedding, inadvertent infection is rare[14] (for example, there are eleven cases of chickenpox which may have been transmitted by vaccinated individuals out of approximately fifty million doses),[15] and only a single case of transmission of influenza virus has been documented, and that person remained asymptomatic.[16]

The attenuated virus from vaccines is much weaker and less likely to infect than the wild virus.[17]

In dogs, vaccine-induced viral shedding has been observed with live attenuated Canine parvovirus vaccines.[18]


Regulatory authorities in the USA and EU recommend that shedding data should be collected both during the development phase of a new product and during clinical trials. A full report on shedding must subsequently be included in the Biologics license application.[19][20]

A study of 345 participants aged 5–49 years who received the live influenza virus vaccine LAIV3, and for whom shedding was assessed by viral culture of nasal swabs (daily for days 1–7 post vaccination, every other day for days 9 through 25, and on day 28) indicated that 30 percent had detectable virus in nasal secretions obtained by nasal swabbing. In an open-label study of 200 children aged 6–59 months who received a single dose of LAIV3, shedding of low titers of at least one vaccine virus was detected on culture in 79 percent of children and was more common among the younger recipients. Serious illnesses have not been reported among unvaccinated persons infected inadvertently with vaccine viruses. The estimated probability of transmission of vaccine virus within a contact group with a single LAIV recipient in this population was 0.58 percent.[21]

Members of the anti-vaccination movement have propagated misinformation claiming that those inoculated with COVID-19 vaccines (which have largely used mRNA or viral vectors) shed spike proteins that can affect the fertility and menstruation of women who are exposed to them.[22][23][24][25][26]


  1. ^ Vaccine Rejection and Hesitancy: A Review and Call to Action. Open Forum Infect Dis. 2017 Summer; 4(3) (Report). 18 July 2017. doi:10.1093/ofid/ofx146.
  2. ^ Anti-vaccine activists, Web 2.0, and the postmodern paradigm – an overview of tactics and tropes used online by the anti-vaccination movement. Epub 2011 Dec 13. "Vaccine", 2012 May 28;30(25):3778–89. (Report). 13 December 2011. doi:10.1016/j.vaccine.2011.11.112.
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  13. ^ Anderson, Evan J (October 2008). "Rotavirus vaccines: viral shedding and risk of transmission". The Lancet Infectious Diseases. 8 (10): 642–649. doi:10.1016/s1473-3099(08)70231-7. ISSN 1473-3099. PMID 18922486.
  14. ^ King, James C.; Treanor, John; Fast, Patricia E.; Wolff, Mark; Yan, Lihan; Iacuzio, Dominic; Readmond, Bernard; O'Brien, Diane; Mallon, Kenneth (2000-02-01). "Comparison of the Safety, Vaccine Virus Shedding, and Immunogenicity of Influenza Virus Vaccine, Trivalent, Types A and B, Live Cold-Adapted, Administered to Human Immunodeficiency Virus (HIV)-Infected and Non-HIV-Infected Adults". The Journal of Infectious Diseases. 181 (2): 725–728. doi:10.1086/315246. ISSN 0022-1899. PMID 10669363.
  15. ^ "vaccine-shedding – The Immunization Partnership". Retrieved 2018-04-29.
  16. ^ Tosh, Pritish K.; Boyce, Thomas G.; Poland, Gregory A. (January 2008). "Flu myths: dispelling the myths associated with live attenuated influenza vaccine". Mayo Clinic Proceedings. 83 (1): 77–84. doi:10.4065/83.1.77. ISSN 0025-6196. PMID 18174020.
  17. ^ "Childhood flu programme: information for healthcare practitioners". GOV.UK. Retrieved 2018-04-29.
  18. ^ Calatayud O, Esperón F, Cleaveland S, Biek R, Keyyu J, Eblate E, Neves E, Lembo T, Lankester F (14 June 2019). "Carnivore Parvovirus Ecology in the Serengeti Ecosystem: Vaccine Strains Circulating and New Host Species Identified". Journal of Virology. 93 (13): e02220-18. doi:10.1128/JVI.02220-18. PMC 6580958. PMID 30996096.
  19. ^ "Design and Analysis of Shedding Studies for Virus or Bacteria-Based Gene Therapy and Oncolytic Products; Guidance for Industry | FDA". Retrieved 2021-01-11.
  20. ^ Longhurst, Sharon. "CURRENT REGULATORY THINKING FOR VIRAL SHEDDING STUDIES IN THE EUROPEAN UNION" (PDF). European Medicines Agency. Retrieved 11 January 2021.
  21. ^ "Safety of Influenza Vaccines – Safety of Inactivated Influenza Vaccines (IIVs)". 24 August 2017. Retrieved 3 February 2021.
  22. ^ Johansen, Nicholas (2021-04-22). "Kelowna store bans anyone who has received COVID-19 vaccine". Castanet. Retrieved 2021-05-11.
  23. ^ "Debunking the anti-vaccine hoax about 'vaccine shedding'". PolitiFact. Retrieved 2021-05-11.
  24. ^ Hannon, Elliot (2021-04-27). "Miami Private School Informs Parents Vaccinated Teachers "May Be Transmitting Something From Their Bodies"". Slate Magazine. Retrieved 2021-05-08.
  25. ^ Mazzei, Patricia (2021-04-26). "A private school in Miami, citing false claims, bars vaccinated teachers from contact with students". The New York Times. ISSN 0362-4331. Retrieved 2021-05-11.
  26. ^ "The Latest Anti-Vax Myth: 'Vaccine Shedding'". MedPage Today. 2021-04-29. Retrieved 2021-05-11.