Circulating vaccine-derived poliovirus
On very rare occasions, if a population is seriously under-immunized, there are enough susceptible children for the excreted vaccine-derived polioviruses to begin circulating in the community. If the vaccine-virus is able to circulate for a prolonged period of time uninterrupted, it can mutate and, over the course of 12-18 months, reacquire neurovirulence. These viruses are called circulating vaccine-derived polioviruses (cVDPV).
The lower the population immunity, the longer these viruses survive. The longer they survive, the more they replicate, change, and exchange genetic material with other enteroviruses as they spread through a community.
If a population is fully immunized against polio, it will be protected against the spread of both wild and vaccine strains of poliovirus.
Episodes of circulating vaccine-derived poliovirus are rare. Over the past ten years – a period during which more than 10 billion doses of oral polio vaccine were given worldwide – cVDPV outbreaks resulted in fewer than 800 cases cases. In the same period, in the absence of vaccination with OPV, more than 6.5 million children would have been paralysed by wild poliovirus.