Filling measles immunity gaps among children participating in a school vaccination screening and catch-up vaccination pilot program in Zambia: school-based cross-sectional serosurvey study

School vaccination screening (SVS) programs can facilitate catch-up vaccination among children who have missed routine immunization, but assessing vaccination status in school-aged children is challenging because vaccination records are often unavailable and caregiver recall may be unreliable. We conducted a school-based cross-sectional serosurvey nested within an SVS program in Ndola District, Zambia, from March to July 2025 to estimate measles seroprevalence and evaluate the relationship between serology and vaccination status ascertained through vaccination cards and caregiver recall. Caregiver-reported measles-containing vaccine (MCV) history was collected, and dried blood spots were tested for measles IgG antibodies by ELISA; children aged <9 years were additionally tested for rubella IgG antibodies.Of 964 eligible children, 924 (95.8%) participated. Nearly one-third lacked a readable vaccination card, primarily children aged ≥5 years. Based on vaccination cards or caregiver recall, 85.2% had received ≥1 routine MCV dose, while 14.8% had received no routine MCV doses; 72.6% reported at least one MCV dose received outside routine services. Overall measles seroprevalence was 85.6% and was higher among children with vaccination documented by card than caregiver recall. Among children aged <9 years classified by caregiver recall as having received no routine MCV doses, 83.6% were seropositive for both measles and rubella, suggesting misclassification of vaccination status. Screening based on vaccination cards alone would have referred 488 children and identified 69.2% (n=93) of measles-seronegative children requiring catch-up vaccination. Adding caregiver recall reduced referrals to 353 children but identified 53.3% (n=71), while including non-routine doses recall further reduced referrals to 194 children and identified 31.5% (n=42). Caregiver recall was more prone to misclassification than vaccination cards, highlighting the need to balance sensitivity and efficiency when designing SVS strategies. Strengthening vaccination documentation systems and clarifying the role of caregiver recall will be important for optimizing school-based vaccination screening programs.​