[Comment] Polio outbreaks due to vaccine-derived viruses demand a re-definition of vaccine safety

Vaccine safety is regarded as the absence of any serious adverse event following immunisation (sAEFI) in the vaccinated individual. The live oral poliovirus vaccine (OPV) causes an sAEFI—vaccine-associated paralytic polio (VAPP)—at a sufficiently low frequency (1 per 100 000 to 750 000 infants vaccinated with one or more doses), resulting in WHO declaring OPV to be safe for wide use.1,2 Thus, WHO, in effect, modified the standard definition of vaccine safety to be the absence of any sAEFI at an unacceptable frequency, thus introducing a subjective, arbitrary, element into the definition.