The Oral Polio Vaccine (OPV) has been a significant tool in the global effort to eradicate poliomyelitis, a debilitating disease. Administered orally, typically as drops, it has played a central role in public health campaigns worldwide. Its development and widespread use marked a turning point in controlling polio, a disease that once caused widespread paralysis and death, particularly in children.
How OPV Confers Immunity
The OPV works by introducing a weakened, or attenuated, form of the live poliovirus into the body. This attenuated virus retains its capacity to stimulate an immune response without causing serious illness. Upon oral administration, the weakened poliovirus replicates in the gut, mimicking a natural infection without causing the paralytic disease associated with wild poliovirus. This replication triggers the body’s immune system to produce antibodies.
The immune response generated by OPV is twofold, involving both humoral and mucosal immunity. Humoral immunity involves the production of antibodies in the bloodstream that can neutralize the virus if it attempts to invade the nervous system. Mucosal immunity, developed in the gut where the virus replicates, is key. This localized immunity helps prevent the wild poliovirus from multiplying in the intestines and being shed in feces, thereby reducing its transmission to others.
OPV’s Role in Polio Eradication
OPV has been instrumental in the near eradication of polio due to several advantages. Its oral administration eliminates the need for trained medical personnel and sterile needles, making it easy to deploy in large-scale vaccination campaigns, even in remote areas. This ease of use also contributes to its lower cost, making it an affordable option for mass immunization programs globally, often costing less than US$0.15 per dose.
A key benefit of OPV is its ability to induce gut immunity, which not only protects the vaccinated individual but also reduces the circulation of the virus within communities. For several weeks after vaccination, the vaccine virus replicates in the intestine and is excreted, potentially providing “passive” immunization to unvaccinated close contacts through environmental spread, further limiting wild poliovirus transmission. Since 1988, global efforts utilizing OPV have reduced wild poliovirus cases by over 99.9%, showing its major impact on public health.
Different Forms of OPV
The development of OPV has evolved to address the changing landscape of poliovirus types. Initially, trivalent OPV (tOPV) contained weakened forms of all three poliovirus serotypes (types 1, 2, and 3). This formulation effectively reduced all three types of wild poliovirus.
Following the eradication of wild poliovirus type 2 in 1999, the global strategy shifted. In April 2016, all countries transitioned from tOPV to bivalent OPV (bOPV), targeting only poliovirus types 1 and 3. This change occurred because the type 2 component of tOPV was responsible for many vaccine-derived poliovirus cases after wild type 2 was no longer circulating. Monovalent OPVs (mOPVs), targeting a single serotype, are reserved for specific outbreak responses. More recently, novel OPVs (nOPVs), such as nOPV2, have been introduced. These newer versions are genetically modified to be more stable, reducing the likelihood of reversion to a virulent form and minimizing risks associated with the vaccine.
Safety Considerations for OPV
While generally safe and effective, OPV carries some rare associated risks. One is vaccine-associated paralytic poliomyelitis (VAPP), where the weakened vaccine virus can revert to a form capable of causing paralysis in the vaccinated individual or their close contacts. This occurs in approximately 1 in 2.7 million doses administered, with most cases occurring after the first dose in recipients or in unvaccinated contacts.
Another risk is the emergence of circulating vaccine-derived polioviruses (cVDPVs). These occur in areas with low immunization coverage, where the attenuated vaccine virus can circulate for extended periods, mutate, and regain neurovirulence, leading to outbreaks. Type 2 cVDPVs have been a primary concern, accounting for most such cases after the removal of the type 2 component from routine OPV. Despite these rare occurrences, OPV’s benefits in preventing widespread polio outweigh these risks, contributing to global disease control.