Polio is a severe infectious disease caused by the poliovirus, which can lead to lifelong paralysis. While global efforts have significantly reduced wild poliovirus cases, a rare phenomenon known as vaccine-derived polio can occur. This article explains vaccine-derived polio and its role in global eradication efforts.
Understanding Polio and Its Vaccines
Polio is a contagious disease caused by the poliovirus, primarily affecting the nervous system. The virus typically enters the body through the mouth, often from contaminated food or water, and replicates in the intestine. In a small percentage of cases, it can invade the central nervous system, leading to permanent muscle weakness and paralysis. Effective vaccines have been crucial in controlling its spread.
Two main types of polio vaccines are used globally. The Oral Polio Vaccine (OPV) contains live, weakened poliovirus. Administered as drops, it mimics natural infection, stimulating intestinal and systemic immunity. The Inactivated Polio Vaccine (IPV) contains killed poliovirus and is given as an injection. IPV primarily induces a strong antibody response in the bloodstream, protecting against paralytic polio.
How Vaccine-Derived Polio Occurs
Vaccine-derived poliovirus (VDPV) emerges from the live, weakened viruses in the Oral Polio Vaccine (OPV). After OPV administration, these weakened viruses replicate in the intestines and are shed in feces. This shedding helps spread immunity in communities with poor sanitation, providing broad protection, including “passive” immunization of unvaccinated contacts.
If the vaccine virus circulates in under-immunized populations for an extended period, it can undergo genetic changes. These changes can cause the weakened virus to revert to a form that can cause paralysis, similar to wild poliovirus. This sustained transmission allows the virus to continuously replicate and evolve. The emergence of VDPV is a consequence of low population immunity, not a flaw in the vaccine itself.
Characteristics and Incidence
Vaccine-derived polio presents with symptoms similar to wild poliovirus, including sudden onset of flaccid paralysis. Its neurological impact is indistinguishable from wild poliovirus infection, as both affect motor neurons. However, genetic analysis of the isolated virus confirms its vaccine-derived origin, which is crucial for tracking outbreak sources.
The occurrence of vaccine-derived polio is extremely rare, considering billions of OPV doses administered worldwide. Approximately one case may occur for every million children vaccinated with OPV. These cases almost exclusively arise in areas with very low polio vaccination coverage, often due to conflict, displacement, or systemic healthcare challenges.
Global Polio Eradication and Vaccine Strategies
Global polio eradication efforts have primarily relied on the Oral Polio Vaccine (OPV) due to its ability to induce gut immunity and block virus transmission. OPV has been highly effective, reducing wild poliovirus cases by over 99% since 1988. However, the rare occurrence of vaccine-derived poliovirus (VDPV) necessitates a strategic shift in the final stages of eradication.
The World Health Organization (WHO) and partners are implementing a phased withdrawal of OPV, transitioning towards exclusive use of the Inactivated Polio Vaccine (IPV). This strategy aims to eliminate the risk of VDPV once wild poliovirus circulation has ceased. IPV provides individual protection against paralysis without the risk of reversion or shedding, making it suitable for maintaining a polio-free world and sustaining gains against wild polio.