Clinical and Epidemiological Investigation of Vaccine-Derived Poliovirus Type 2 Outbreak in Pakistan During 2019–2021

BACKGROUND: To mitigate the risk of circulating vaccine-derived poliovirus type 2 (cVDPV2) establishment and associated paralytic cases, oral polio vaccine 2 was globally withdrawn from the routine immunization schedule in 2016, soon after the certification of wild poliovirus type 2 eradication. We investigated the epidemiology of cVDPV2 outbreak and impact of type 2 immunization response in Pakistan to contain the transmission of poliovirus type 2 after trivalent to bivalent oral polio vaccine switch in 2016. METHODS: Epidemiological, virological, and immunization data were assessed to ascertain the effectiveness of cVDPV2 outbreak response activities. A total of 35 724 paralytic cases and 2804 sewage wastewater samples collected between July 2019 and March 2022 were tested for cVDPV2 detection. RESULTS: Circulating vaccine-derived poliovirus type 2 was identified in 0.5% (181/35 724) of paralytic cases and 11% (298/2804) of sewage wastewater samples. The cVDPV2 strains were grouped into 13 indigenous and 2 imported emergence groups. Fourteen vaccination rounds of oral poliovirus type 2 (monovalent oral polio vaccine/trivalent oral polio vaccine) and 5 rounds of inactivated poliovirus vaccine were conducted between September 2019 and December 2021, resulting in successful interruption of cVDPV2 transmission in ∼2 years. CONCLUSIONS: Our findings highlight the imperative need of optimal vaccination coverage during routine immunization to build up and sustain the immunity against poliovirus for successful eradication of endemic wild poliovirus type 1 and to prevent cVDPV2 emergence.