Recurring Outbreaks of Circulating Vaccine-derived Polioviruses: Implications for Global Poliovirus Immunization Strategy.

The incidence of polio has decreased by more than 99.9% and currently, only two countries are endemic for wild poliovirus. However, increasing outbreaks of circulating vaccine-derived poliovirus globally in the last few years, with the latest ones in high-income, exclusive inactivated polio virus vaccine (IPV)-using countries have brought out a new dimension to the end game of polio eradication. The inability of the current IPV to induce efficient mucosal immunity in the intestine is likely to be one of the key reasons behind the silent transmission of the polio virus in these countries. New challenges demand concerted global efforts with renewed vigor to cross the last mile. We need to aggressively cover up areas of under-vaccination and continue large-scale genomic surveillance. Further, the availability of a novel oral polio vaccine (nOPV2), and the likely availability of Sabin IPV and a more refined IPV with mucosal adjuvant in the near future is likely to go a long way in achieving this remarkable feat.