Vaccine shedding is a form of

viral shedding Viral shedding is the expulsion and release of virus progeny following successful reproduction during a host cell infection. Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruse ...

which can occur following a viral infection caused by an attenuated (or "live virus") vaccine, which is a specific

vaccine A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified.

technology that uses an attenuated form of a live virus. Illness in others resulting from transmission through this type of viral shedding is rare. A large proportion of vaccines are not attenuated (live virus) vaccines, and therefore cannot cause vaccine-induced viral shedding. The specific use of the term "vaccine shedding" has risen to public prominence through

anti-vaccine Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain abou ...

activists linked to misinformation related to COVID-19, who erroneously claim that COVID-19 vaccination can cause individuals to shed

coronavirus spike protein Spike (S) glycoprotein (sometimes also called spike protein, formerly known as E2) is the largest of the four major structural proteins found in coronaviruses. The spike protein assembles into trimers that form large structures, called spi ...

and affect menstruation and fertility in women exposed to them. However, the spike protein generated by vaccination does not shed, and there is no evidence to suggest that these vaccines cause menstruation and fertility problems. Vaccination also cannot cause shedding of the COVID-19 virus since none of the

COVID-19 vaccines A COVID19 vaccine is a vaccine intended to provide acquired immunity against severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), the virus that causes coronavirus disease 2019 (COVID19). Prior to the COVID19 pandemic, an est ...

authorized for use by the

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

or the

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of ...

as of December 2021 are live-virus vaccines. Despite this, a

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease quickly ...

"vaccine shedding" conspiracy theory has subsequently emerged, leading to

vaccine hesitancy Vaccine hesitancy is a delay in acceptance, or refusal, of vaccines despite the availability of vaccine services and supporting evidence. The term covers refusals to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain abou ...

among some people. Shedding is only possible with an attenuated vaccine. It is impossible with other vaccine technologies such as

inactivated vaccine An inactivated vaccine (or killed vaccine) is a vaccine consisting of virus particles, bacteria, or other pathogens that have been grown in culture and then killed to destroy disease-producing capacity. In contrast, live vaccines use pathogens ...

(killed-virus vaccines), viral vector vaccine,

RNA vaccines An mRNA vaccine is a type of vaccine that uses a copy of a molecule called messenger RNA (mRNA) to produce an immune response. The vaccine delivers molecules of antigen-encoding mRNA into immune cells, which use the designed mRNA as a blueprin ...

(that contain no virus), 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. With the exception of the oral polio vaccine (OPV), there have been no documented cases of vaccine-induced viral shedding that has infected contacts of a person vaccinated with an attenuated (live-virus) vaccine.

Occurrences

The only human vaccine to have caused any significant number of infections is the oral polio vaccine (OPV), which takes advantage of the contact immunity from shed vaccine virus to amplify the effectiveness of vaccination campaigns. However, under conditions of persistent circulation in undervaccinated populations, reversion mutations that reactivate the virus can give rise to disease-causing circulating vaccine-derived poliovirus (cVDPV) strains, and this vaccine is no longer used in developed countries. However, the OPV is still used in Asia and Africa, leading to a small number of vaccine-induced polio infections each year. The route of infection is through contact with

faeces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

, and some live vaccines, like the viruses they prevent, are shed in stool for up to 28 days. Normal hygiene is sufficient to prevent infection among healthy individuals, but

immunocompromise Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

d individuals need to be especially diligent. Other attenuated vaccines show no significant viral shedding, and inadvertent infection is rare. For example, only eleven cases of chickenpox transmission by vaccinated individuals have been documented out of approximately fifty million doses, and but a single documented case of

influenza virus ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ' ...

transmission, and that person remained asymptomatic. The attenuated virus from vaccines is much weaker and less likely to infect than the wild virus. In dogs, vaccine-induced viral shedding has been observed with live attenuated

Canine parvovirus Canine parvovirus (also referred to as CPV, CPV2, or parvo) is a contagious virus mainly affecting dogs. CPV is highly contagious and is spread from dog to dog by direct or indirect contact with their feces. Vaccines can prevent this infection ...

vaccines.

Research

Regulatory authorities in the US and EU recommend that shedding data should be collected both during the development phase of a new product and during

clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...

s. A full report on shedding must subsequently be included in the

Biologics license application A biologics license application (BLA) is defined by the U.S. Food and Drug Administration (FDA) as follows: The biologics license application is a request for permission to introduce, or deliver for introduction, a biologic product into inters ...

. 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 days9 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.

References

{{Vaccines Vaccines Virology