High measles and rubella vaccine coverage and seroprevalence among Zambian children participating in a measles and rubella supplementary immunization activity.

Post-campaign coverage surveys estimate the proportion of previously unvaccinated children vaccinated during supplementary immunization activities (SIA) but cannot address whether susceptible children seronegative to measles and rubella viruses were vaccinated during the SIA. We conducted a cross-sectional serosurvey nested within the November 2020 measles-rubella SIA in Zambia, systematically sampling children aged 9 months to 5 years at 30 SIA sites in Choma and Ndola Districts. A questionnaire administered to caregivers collected the child's demographic information and vaccination history. Dried blood spot specimens were collected from child participants and for Immunoglobulin-G antibodies to measles and rubella viruses by enzyme immunoassay. We used the expected vaccination probability by age derived from Demographic and Health Surveys to quantify the value of the immunization campaign, estimating the proportion of children who would not have otherwise received routine MR doses. Among children enrolled with MR vaccination data (N = 2,364), 2,214 (94%) reported at least one routine MR dose before the SIA. We estimate 5.0% [95%CI, 4.2-5.9%] (n = 118/2364) of children would not have otherwise received routine MR dose 1 without the SIA and 23.4% [95%CI, 21.7-25.1] (n = 553/2364) would not have otherwise received routine MR dose 2. Thus, 1 in 3.5 doses were given to an MR un- or under-vaccinated child who may not have received that dose in the absence of an SIA. Eighty-six percent and 90% of children were measles and rubella seropositive before vaccination during the SIA. Thirty-six percent of children with no prior routine MR dose were measles seropositive, while nine percent of children with two prior routine MR doses were measles seronegative. Although children vaccinated during this SIA were highly likely to have previously received routine vaccinations, the SIA reached a considerable number of susceptible children. Monitoring SIA effectiveness and efficiency through standardized metrics and formats is critical for impactful vaccine delivery strategies.