The Oral Polio Vaccine (OPV) is a live attenuated vaccine containing Sabin strains of poliovirus (serotypes 1, 2, and 3). It is a cornerstone of the global effort to eradicate poliomyelitis.

1. Mechanism and Immunity

• Administration: Given as oral drops, the vaccine viruses infect and multiply within the intestinal mucosa cells.
• Mucosal Immunity: It induces robust secretory IgA in the gut, which protects the individual from paralytic disease and significantly reduces feco-oral transmission, thereby interrupting the circulation of wild polioviruses.
• Herd Immunity: Vaccinated individuals excrete the attenuated virus, which can spread to and protect unimmunized contacts in the community.

2. Types of OPV

• Trivalent OPV (tOPV): Contained all three serotypes; it was globally replaced by bivalent OPV in 2016 to eliminate the risk associated with the type 2 Sabin strain.
• Bivalent OPV (bOPV): Contains only serotypes 1 and 3. It is currently used in routine immunization.
• Monovalent OPV (mOPV): Contains only one serotype (mOPV1, mOPV2, or mOPV3).
  • mOPV1 and mOPV3 are more immunogenic than trivalent OPV against their respective types and have been used in supplemental immunization campaigns to target specific outbreaks.
  • mOPV2 was used specifically for responding to type 2 outbreaks after the global tOPV-to-bOPV switch.

3. Novel Oral Polio Vaccine (nOPV)

• nOPV2: This is a genetically stabilized variant of mOPV2. It was developed to address outbreaks of circulating vaccine-derived poliovirus type 2 (cVDPV2).
• Advantages: nOPV2 is as safe and effective as Sabin mOPV2 but is significantly less likely to revert to a virulent form that causes paralysis in settings with low population immunity.
• Future Strains: Genetically stable novel strains for serotypes 1 and 3 (nOPV1 and nOPV3) are in development and may eventually replace bOPV.

4. Risks and the "Endgame" Strategy

• VAPP and cVDPV: Rarely, the Sabin strains can cause Vaccine-Associated Paralytic Poliomyelitis (VAPP) in a recipient or mutate into circulating vaccine-derived polioviruses (cVDPV) in areas with low vaccine coverage.
• Sequential Schedule: To mitigate these risks, the current "Polio Endgame" strategy involves a sequential schedule using both Inactivated Polio Vaccine (IPV) and OPV. IPV provides systemic immunity to prevent paralysis, while OPV maintains the mucosal immunity needed to stop virus transmission.

5. Immunization Schedule (India)

• Birth Dose: A "zero dose" is given at birth.
• Primary Series: Three doses of bOPV are administered at 6, 10, and 14 weeks, along with fractional doses of IPV at 6 and 14 weeks.
• Boosters: A booster dose of bOPV is typically given at 16–24 months.