Vaccine shedding

Vaccine shedding is a term used for the rare release of virus following administration of a live-virus vaccine. Shedding is a popular anti-vaccination trope,[1][2][3] but, with the exception of the oral polio vaccine (OPV) in the 1950s, there have only been a few documented cases of vaccine-strain virus infecting contacts of a vaccinated person.[4]

Viral shedding is part of the mechanism of virus transmission.[5] Shedding is impossible with killed vaccines or those made using only isolated proteins (most vaccines fall into one of these two classes), but a small number of vaccines contain live attenuated virus which can theoretically infect others. Not all pathogens are shed, shedding does not equal transmission, and transmission does not always cause disease.[6]

The only vaccine to have caused any significant number of infections is the oral polio vaccine (OPV) in the 1950s,[7] and this vaccine is no longer used. The route of infection was through contact with faeces, and some vaccines, like the viruses they prevent, are shed in stool for up to 28 days.[8] Normal hygiene is sufficient to prevent infection[9] but immunocompromised individuals need to be especially diligent.[10] Other attenuated vaccines show no significant shedding, inadvertent infection is rare[11] (for example, there are 11 cases of chickenpox which may have been transmitted by vaccinated individuals out of approximately 50 million doses),[12] and only a single case of transmission of influenza virus has been documented, and that person remained asymptomatic.[13] The attenuated virus from vaccines is much weaker and less likely to infect than the wild virus.[14]