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Background Since July 2019, Pakistan and Afghanistan have been facing an outbreak of serotype-2 circulating vaccine derived poliovirus (cVDPV2) in addition to continued transmission of serotype-1 wild poliovirus (WPV1) and SARS-CoV-2 in 2020. Understanding the risks of cVDPV2 transmission due to pause of global vaccination efforts and the impact of potential vaccination response strategies in the current context of COVID-19 mitigation measures is critical. Methods We developed a stochastic, geographically structured mathematical model of cVDPV2 transmission which captures both mucosal and humoral immunity separately and allows for reversion of serotype-2 oral polio vaccine (OPV2) virus to cVDPV2 following vaccine administration. The model includes geographic heterogeneities in vaccination coverage, population immunity and population movement. The model was fitted to historic cVDPV2 cases in Pakistan and Afghanistan between January 2010-April 2016 and July 2019-March 2020 using iterated particle filtering. The model was used to simulate spread of cVDPV2 infection from July 2019 to explore impact of various proposed vaccination responses on stopping transmission and risk of spread of reverted Sabin-2 under varying assumptions of impacts from COVID-19 lockdown measures on movement patterns as well as declines in vaccination coverage. Results Simulated monthly incidence of cVDPV2 from the best-fit model demonstrated general spatio-temporal alignment with observed cVDPV2 cases. The model predicted substantial spread of cVDPV2 infection, with widespread transmission through 2020 in the absence of any vaccination activities. Vaccination responses were predicted to substantially reduce transmission and case burden, with a greater impact from earlier responses and those with larger geographic scope. While the greatest risk of seeding reverted Sabin-2 was predicted in areas targeted with OPV2, subsequent spread was greatest in areas with no or delayed response. The proposed vaccination strategy demonstrated ability to stop the cVDPV2 outbreak (with low risk of reverted Sabin-2 spread) by February 2021. Conclusion Outbreak response vaccination campaigns against cVDPV2 will be challenging throughout the COVID-19 pandemic but must be implemented urgently when feasible to stop transmission of cVDPV2.

BACKGROUND: Stopping serotype 1 wild poliovirus transmission in Pakistan and Afghanistan requires ensuring all children 90% across HRMP type, ethnic group, language and mobility means. CONCLUSION: Large numbers of HRMPs were found across Pakistan, with substantial links throughout the country and with Afghanistan. While vaccination coverage of HRMPs was high, ensuring these populations are consistently vaccinated remains a priority.

The first large-scale vaccination campaign using needle-free jet injectors to administer fractional doses of inactivated poliovirus vaccine (fIPV) was conducted in Karachi, Pakistan, in February 2019. Data on acceptability of jet injectors were collected from 610 vaccinators and 4898 caregivers during the first four days of the campaign. Of those with prior needle and syringe experience, both vaccinators and caregivers expressed a strong preference for jet injectors (578/592 [97.6%] and 4792/4813 [99.6%], respectively), citing ease of use, appearance, and child's response to vaccination. Among caregivers, 4638 (94.7%) stated they would be more likely to bring their child for vaccination in a future campaign that used jet injectors. Mean vaccine coverage among towns administering fIPV was 98.7% - an increase by 18.4% over the preceding campaign involving full-dose IPV. Our findings demonstrate the strong acceptability of fIPV jet injectors and highlight the potential value of this method in future mass campaigns.