Vaccine-derived polioviruses (VDPVs) are rare strains of poliovirus that have genetically mutated from the strain contained in the oral polio vaccine. The oral polio vaccine contains a live, attenuated(weakened) vaccine-virus. When a child is vaccinated,the weakened vaccine-virus replicates in the intestine and enters into the bloodstream, triggering a protective immune response in the child. Like wild poliovirus, the child excretes the vaccine-virus for a period of six to eight weeks. Importantly, as it is excreted, some of the vaccine-virus may no longer be the same as the original vaccine-virus as it has genetically altered during replication. This is called a VDPV.
Types of VDPVs
1. Circulating vaccine-derived poliovirus (cVDPV)
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. Between 2000 and 2011 – a period in which more than 10 billion doses of oral polio vaccine were given worldwide – 20 cVDPV outbreaks occurred, resulting in 580 polio cases. In the same period, in the absence of vaccination with OPV, around 6 million children would have been paralysed by poliovirus.
2. Immunodeficiency-related vaccine-derived poliovirus (iVDPV)
Prolonged replication of VDPVs has been observed in a small number of people with rare immune deficiency disorders. Because they are not able to mount an immune response, these people are not able to clear the intestinal vaccine virus infection, which is usually cleared within six to eight weeks. They therefore excrete iVDPVs for prolonged periods.
The occurrence of iVDPVs is very rare. Only 111 cases have been documented worldwide since 1962. Of these, most stopped excretion within six months or died.
3. Ambiguous vaccine-derived poliovirus (aVDPV)
aVDPVs are VDPVs that are either isolated from people with no known immunodeficiency, or isolated from sewage whose ultimate source is unknown. Very little is known about them.
Implications and management of vaccine-derived polioviruses
Circulating vaccine-derived polioviruses must be managed in the same way as wild poliovirus outbreaks. The solution is the same for all polio outbreaks: vaccinate every child several times with oral polio vaccine to stop polio transmission, regardless of whether the virus is wild or vaccine-derived.
Vaccine-derived polioviruses appear to be less transmissible than wild poliovirus. Outbreaks are usually self-limiting or rapidly stopped with 2–3 rounds of high-quality supplementary immunization activities.
Once wild poliovirus transmission has been stopped globally, the vaccine-viruses will be the only source of live polioviruses in the community and could potentially lead to the re-emergence of polio. Use of the oral polio vaccine in routine immunization programmes will therefore be phased out to eliminate the rare risks posed by vaccine-derived polioviruses.