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Why can the oral poliovirus vaccine indirectly protect other people in a community?

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Polio vaccine — polio A health care worker giving a polio vaccine to a child in Katsina state, Nigeria, 2014.

The oral poliovirus vaccine (OPV) can indirectly protect others in a community because it contains a live, attenuated (weakened) form of the virus. After a person receives the vaccine, this weakened virus multiplies in their small intestine and is subsequently shed through their feces.12

As the virus is shed, it can spread to other individuals in the community through the fecal-oral route. When these individuals are exposed to the weakened virus, it can stimulate their own immune systems to generate antibodies against the poliovirus, thereby providing them with immunity. This process helps to boost herd immunity within a population, which is particularly useful in areas where the virus is still actively circulating.1324

While this mechanism can be beneficial for community-wide protection, it is important to note that in rare instances, the weakened virus can mutate or revert to a more infectious form, potentially causing vaccine-associated paralytic polio or circulating vaccine-derived polioviruses in under-immunized populations.124