Insights on vaccine-derived poliovirus type 2 outbreaks and novel oral polio vaccine candidates.

Despite the life-saving benefits of the vaccination with the live attenuated oral polio vaccine, or Sabin (OPV), OPV still evolves rapidly to lose attenuating determinants critical to the reacquisition of virulence, resulting in vaccine-derived, virulent poliovirus variants. The circulation of these variants within under-immunized populations leads to the further evolution of circulating, vaccine-derived poliovirus with increased transmission capacity, representing a significant risk of polio re-emergence. Although the wild poliovirus type 2 (WPV2) had been declared as eradicated worldwide in 2015, outbreaks of circulating VDPV-type 2 (cVDPV2) have been increasingly reported, especially after the tOPV-bOPV switch that occurred in 2016, which resulted in building a cohort of susceptible individuals with lower type 2 intestinal immunity. Since the response to outbreaks is done through conducting supplementary immunization activities (SIAs) using type-2 containing OPV, this carries a risk of mutations, especially with suboptimum coverage and potential further circulation. The recent detection of outbreaks of cVDPV2 in some countries necessitated the development of a genetically stable novel oral polio vaccine type 2 (nOPV-2) to decrease the chances of mutations and hence outbreaks. This review highlights the recent outbreaks of VDPV serotypes, emphasizing the role of the World Health Organization and the Global Polio Eradication Initiative in detecting and controlling VDPV outbreaks. The emerging VDPV, strategies implemented, reported clinical trials, and proposals for developing nOPV. The way the review is presented will encourage assertiveness, assist in posing open-ended questions, and provide several ideas for further research.