The Oral Polio Vaccine (OPV) is a liquid vaccine administered by drops into the mouth to prevent poliomyelitis, a paralyzing disease caused by the poliovirus. Developed by Albert Sabin, OPV has been instrumental in reducing global polio cases by over 99% since its introduction in the 1960s. The vaccine’s low cost, ease of administration, and effectiveness made it central to the Global Polio Eradication Initiative established in 1988 to protect children and stop the spread of the virus worldwide.
How the Oral Vaccine Creates Immunity
The oral vaccine relies on a live, weakened (attenuated) version of the poliovirus. When administered, this virus replicates briefly in the gut, the primary site of natural infection. This replication mimics a natural infection without causing disease, prompting the immune system to build a defense.
Crucially, OPV generates a strong local defense called mucosal immunity within the intestinal tract. This defense is primarily mediated by secretory Immunoglobulin A (IgA) antibodies, which coat the lining of the gut. This intestinal immunity blocks the wild poliovirus from replicating in the gut and prevents the vaccinated person from shedding the virus in their feces, thereby stopping community transmission.
OPV Versus the Inactivated Polio Vaccine
The Oral Polio Vaccine (OPV) differs fundamentally from the Inactivated Polio Vaccine (IPV), which is administered by injection. OPV contains live, attenuated strains, while IPV contains poliovirus killed with formalin. Both vaccines induce systemic immunity by creating antibodies in the bloodstream that protect against paralytic polio.
OPV is highly effective at inducing mucosal immunity in the gut, preventing the virus from replicating and spreading through the community. IPV primarily induces systemic immunity but provides only limited mucosal protection. While protected from paralysis, an individual vaccinated only with IPV can still shed the virus from their gut, potentially infecting others in areas with poor sanitation.
IPV is the standard vaccine in countries where wild polio has been eliminated, as the risk of transmission is low. OPV remains a primary tool in endemic and outbreak regions due to its superior ability to interrupt person-to-person spread. Its ease of oral administration and low cost (approximately $0.15 to $0.20 per dose) make OPV suitable for large-scale, rapid-response vaccination campaigns.
Understanding Vaccine-Derived Poliovirus
A specific and rare risk associated with the use of live OPV is the potential for the weakened vaccine virus to mutate. As the attenuated virus replicates in the intestine, it can genetically change and, in rare instances, revert to a form capable of causing paralysis. This genetically altered strain is referred to as a vaccine-derived poliovirus (VDPV).
This phenomenon involves two main concerns: Vaccine-Associated Paralytic Poliomyelitis (VAPP) and circulating Vaccine-Derived Poliovirus (cVDPV). VAPP is a single, non-spreading case of paralysis occurring in the recipient or a close, unvaccinated contact (about one case per 2.7 million doses). In contrast, a cVDPV emerges when the reverted virus circulates among a population with low immunization coverage, allowing it to mutate further and regain the ability to cause paralytic outbreaks.
The majority of cVDPV outbreaks have historically been caused by poliovirus Type 2, which was eliminated as a wild virus in 1999. The risk of cVDPV emergence is the main reason why OPV has been phased out of routine use in non-endemic countries.
The Current Global Use of OPV
The use of OPV is strategically managed as part of the Global Polio Eradication Initiative, focusing on high-risk and outbreak settings. Following the eradication of wild poliovirus Type 2, a coordinated switch occurred in 2016 from trivalent OPV (tOPV, protecting against all three types) to bivalent OPV (bOPV, targeting Types 1 and 3). This switch eliminated the risk of Type 2 cVDPV emergence from routine immunization.
Outbreaks of Type 2 cVDPV have continued to emerge in under-immunized populations, requiring a specific response tool. The novel Oral Polio Vaccine Type 2 (nOPV2) was developed to address this; it is genetically modified to be more stable and less likely to revert to a virulent form. This next-generation vaccine is deployed for outbreak response under emergency use authorization and has shown a trend toward decreasing the intensity and emergence of cVDPV2 outbreaks.
The long-term strategy for polio eradication involves the eventual complete cessation of all OPV use globally. Until then, countries are using a mixed schedule, often including at least one dose of IPV before OPV administration, to boost systemic immunity and reduce the risk of VAPP. This phased approach maintains high population immunity while eliminating the risk posed by the live vaccine virus.