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).

Background: The current polio epidemiology in Pakistan poses a unique challenge for global eradication, with polio transmission dynamics influenced by regional variations in immunity and disparities in immunization coverage. This study assesses the immunity level for all three poliovirus types among children aged 6–11 months in polio high-risk regions of Pakistan. Methods: Four consecutive rounds of cross-sectional serological surveys were conducted in polio high-risk areas of Pakistan between November 2016 and October 2023. Twelve high-risk areas were covered in the first three rounds of the survey, while 44 high-risk areas were covered in the fourth round. 25 clusters from each geographical stratum were selected utilizing probability proportional to size. Results: Across the four rounds of the survey, 32,907 children aged 6–11 months from 2084 clusters and 32,371 households were covered. Comparative analysis across the survey rounds showed that seroprevalence of poliovirus type 1 was high in provinces (>95%), albeit consistently lower in Balochistan (going down to 89.7% in Round 4). Type 2 seroprevalence was significantly lower and more heterogeneous, from 34.6% in Sindh to 83.4% in Punjab, with sharp declines by round 4, particularly in Balochistan (40.4%). Type 3 seroprevalence was overall high (>94% in Punjab, Sindh, and KPK) but dropped in the last round, while Balochistan exhibited continually lower immunity (81.1%). Conclusions: The findings reflect the variations in population immunity to poliovirus in the country, with notable fluctuations over the years. The gaps in type 2 immunity over time and consistently lowest in Balochistan highlight the need for continued monitoring of immunity levels and adaptable vaccination strategies.

Background: Measles remains a major cause of morbidity and mortality among children in low- and middle-income countries (LMICs), despite widespread implementation of routine two-dose measles-containing vaccine (MCV) schedules. Serological assessment provides an objective measure of vaccine-induced immunity; however, evidence describing post-vaccination serological protection among children in LMICs remains fragmented and methodologically heterogeneous. Objective: To synthesize evidence on serological protection following routine two-dose MCV among children aged 1–5 years in LMICs and to identify determinants of immunity, methodological limitations, and evidence gaps relevant to immunization programmes. Methods: This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines and prospectively registered in PROSPERO (CRD420261300545). Literature searches were conducted in PubMed, MEDLINE, Scopus, Cochrane Library, CINAHL (EBSCO), and SciSpace on 21 January 2026. Two reviewers independently screened studies, extracted data, and assessed quality of evidence using Joanna Briggs Institute critical appraisal tools. Substantial clinical and methodological heterogeneity precluded meta-analysis; therefore, findings were synthesized narratively. Results: Eighteen studies met the eligibility. These studies were from 14 LMICs across four WHO regions (61.1% African Region, 27.8% Eastern Mediterranean Region, 5.6% each South-East Asia and the region of the Americas). Reported MCV seroprotection following completion of the two-dose schedule generally ranged from 30.6% to 100%, with the highest levels (87–100%), but with evident lower protection in specific settings, such as rural Pakistan (30.6%) and Ethiopia (40–54%) despite having documented vaccination records. Lower protection was consistently associated with undernutrition, human immunodeficiency virus (HIV) infection, incomplete or delayed vaccination, younger age at first dose, and weak programme performance. Considerable variability was observed in study design, vaccination schedules, laboratory assays (ELISA, PRNT, haemagglutination inhibition, and others), seroprotective thresholds, timing of antibody assessment, and participant characteristics. Risk of bias was largely moderate-to-high, and certainty of evidence ranged from low to very low. Conclusions: Routine two-dose measles vaccination generally provides substantial serological protection among children aged 1–5 years in LMICs. Nevertheless, methodological heterogeneity and geographical evidence gaps limit direct comparisons and reduce confidence in cross-study interpretation. Greater standardization of serological assessment and thresholds, harmonized reporting methods, and well-designed longitudinal studies are needed to strengthen the evidence base and support immunization policies aimed at achieving and sustaining measles elimination.

Background: The current polio epidemiology in Pakistan poses a unique challenge for global eradication, with polio transmission dynamics influenced by regional variations in immunity and disparities in immunization coverage. This study assesses the immunity level for all three poliovirus types among children 6-11 months in polio high-risk regions of Pakistan. Methods: Four consecutive rounds of cross-sectional serological surveys were conducted in polio high-risk areas of Pakistan between November 2016 and October 2023. 12 high-risk areas were covered in the first three rounds of the survey, while 44 high-risk areas were covered in the fourth round. 25 clusters from each geographical stratum were selected utilizing probability proportional to size. Results: Across the four rounds of the survey, 32,907 children aged 6–11 months from 2,084 clusters and 32,371 households were covered. Temporal analysis showed that seroprevalence of poliovirus type 1 was high in provinces (>95%), albeit consistently lower in Balochistan (going down to 89.7% in Round 4). Type 2 seroprevalence was significantly lower and more heterogeneous, from 34.6% in Sindh to 83.4% in Punjab, with sharp declines by Round 4, particularly in Balochistan (40.4%). Type 3 seroprevalence was overall high (>94% in Punjab, Sindh, and KPK) but dropped in the last round, while Balochistan exhibited continually lower immunity (81.1%). Conclusion: The findings reflect the variations in population immunity to poliovirus in the country, with notable fluctuations over the years. The gaps in type 2 immunity over time and consistently lowest in Balochistan highlight the need for continued monitoring of immunity levels and adaptable vaccination strategies.