The Global Polio Eradication Initiative (GPEI) welcomes the expert evaluation of the 2016 global “switch” from the use of trivalent oral poliovirus vaccine (tOPV) to bivalent oral poliovirus vaccine (bOPV) in polio eradication efforts. The GPEI commissioned this evaluation to gain a better understanding of the conditions and challenges associated with the switch and its outcome; GPEI will use the learnings to inform the planning that is underway for the eventual cessation of all OPV use, as part of a broad Post-Certification Strategy. We accept the report’s findings of shortcomings in the implementation of the mitigation measures before and after the switch that resulted in an increase in cases and outbreaks.

With the goal of preventing all disease and transmission from type 2 poliovirus, the decision to withdraw type 2-containing OPVs globally was made based on the best available evidence at the time, including guidance from the Strategic Advisory Group of Experts on immunization (SAGE) and endorsement by World Health Assembly (WHA) resolution.

Despite careful evaluation of the risk of variant polio outbreaks, population immunity in some places proved lower than estimated. As a result, outbreaks, particularly in parts of Africa and Asia, grew and spread more than anticipated, which required using type 2 monovalent OPV (mOPV2) to respond. Meanwhile, supply constraints on both inactivated polio vaccine (IPV) and mOPV2 further reduced the scope of responses needed to decisively stop these outbreaks. In addition, sub-optimal quality of the outbreak response campaigns led to continuation of transmission and the emergence of new variant strains.

To ensure successful cessation of all OPVs following eradication of wild poliovirus, the GPEI is applying the lessons from this evaluation.  These lessons have informed the policy framework, triggers and principles of risk tiering for OPV cessation, which have been endorsed by SAGE.  For example, the programme will enhance efforts in historically hard-to-reach places to increase vaccination coverage before OPV cessation. Higher levels of baseline immunity will be a major OPV-cessation success factor.

At the time of the switch, the GPEI established an OPV2 cessation risk task team to monitor and mitigate evolving threats. Since the switch, GPEI partners and affected countries have taken important steps to ensure that any trace of poliovirus transmission is detected quickly, and outbreak responses are fast and effective. These steps are making a positive difference: cases of type 2 variant polio have fallen by approximately 60% from a peak in 2020 to those reported in 2023, and they continue to decline in 2024. However, the remaining presence of variant poliovirus transmission in many places underscores the threat that polio will continue to pose until all transmission is stopped for good.

The GPEI leadership is continuing to engage closely with affected countries to ensure commitments to high-quality outbreak responses, including through stronger engagement of communities. Through its partnership with Gavi, the Vaccine Alliance, the GPEI gives increased focus to strengthening routine immunization, ensuring adequacy of IPV supply and adding a second dose to the essential immunization schedule, which is helping to lay the groundwork for a lasting end to polio.

An innovative tool has been available to address variant poliovirus since 2021 — novel oral polio vaccine type 2 (nOPV2), with over 1.2 billion doses administered to children in 42 countries around the world. Use of nOPV2 helps countries to respond to outbreaks with reduced risk of seeding new variant emergences. Further work to bolster nOPV2 vaccine supply, improve outbreak response time through faster detection and testing, and closing immunity gaps to prepare for OPV cessation will help to achieve and sustain eradication.

Download the expert evaluation report.