Progress and gaps in poliovirus immunity: Evidence from a serological survey of children aged 6-23 months in high-risk districts of Pakistan

Wild poliovirus remains endemic in Pakistan and Afghanistan despite global progress. We quantified immunity to poliovirus types 1-3 among children aged 6-23 months in 44 high-risk districts (2022-2023) using a cross-sectional serosurvey with probability proportional to size (PPS) cluster sampling. We enrolled 20,680 children (10,112 aged 6-11 months; 10,568 aged 12-23 months). Seroprevalence among 6-11-month-olds was 94.5% (type 1), 44.6% (type 2), and 88.0% (type 3); among 12-23-month-olds, it was 95.9%, 53.8%, and 91.2%, respectively. Type 1 seropositivity was highest across provinces; type 3 exceeded 90% except in Balochistan and KP; type 2 was lowest everywhere. Younger children have lower immunity. In multivariable models, residence in Balochistan predicted reduced seroprotection (AOR 0.178, 95% CI 0.066-0.484); older age (AOR 1.356, 1.161-1.583), full immunization (AOR 2.004, 1.643-2.444); and receiving <4 oral poliovirus vaccine (OPV) doses showed higher odds (AOR 1.25, 1.021-1.529) of seroprotection. Wealth showed a non-linear association. Gaps in types 2-3 warrant stronger routine immunization, expanded inactivated polio vaccine (IPV), and tailored supplementary immunization activities (SIAs).