Contributing factors to declining vaccination campaign quality and resurgence of polio cases in Quetta block, Pakistan: a five-year analysis (2020–2024)

Introduction Implementation of supplementary immunization activities (SIAs) delivering oral poliovirus vaccine (OPV) to children under 5 years of age is a key strategy for interruption of poliovirus transmission. Quetta block (Quetta, Pishin, Chaman, and Killa Abdullah districts), which has the largest number of children under 5 years in Balochistan province, is a core reservoir for wild poliovirus type 1 (WPV1) in Pakistan. This study examined the quality of polio SIAs and trends in WPV1 cases in Quetta block during 2020–2024. Methods We conducted descriptive analysis of administrative coverage, lot quality assurance sampling (LQAS) surveys and post-campaign monitoring (PCM) data for polio SIAs implemented during 2020–2024. Data on WPV1 cases and environmental isolates for the same period were obtained from the acute flaccid paralysis (AFP) and environmental surveillance databases. Results Of the 33 SIAs planned in Quetta block during 2020–2024, only 24 (73%) were fully implemented. Due to the COVID-19 pandemic, all polio SIAs in Pakistan were paused in the first half of 2020 but resumed by July 2020. The quality of SIAs in Quetta block declined during the period under review, with the proportion of LQAS lots that achieved the threshold for acceptable performance (≥95%) reducing from 90% in 2020 to 70% in 2023. WPV1 cases re-emerged in 2024 in Quetta block after a three-year absence and a 28-month period without WPV1 isolations from environmental samples. The 14 WPV1 cases that were reported in Quetta block accounted for 19% of all 74 cases in Pakistan and 98% of environmental samples within the block tested positive for WPV1 in 2024. Conclusion Quetta block in Balochistan province experienced a significant reduction in the quality of polio SIAs during 2020–2024, which likely contributed to a resurgence in WPV1 cases in 2024. Factors influencing the decline in SIA quality included COVID-19 disruptions of immunization activities, security incidents, community boycotts, and vaccine hesitancy. To enhance the prospects of poliovirus elimination in Balochistan, stronger government commitment and greater community engagement will help to improve vaccine access and acceptance.