Vaccine shedding

Vaccine shedding is a term used for the 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 been few documented cases of vaccine-strain virus infecting contacts of a vaccinated person.[4]

Viral shedding is part of the normal 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]

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.[15][16]

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% 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% 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%. [17]