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 number and proportion of previously unvaccinated children vaccinated during a supplementary immunization activity (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. Children aged 9 months to 5 years were systematically sampled at 30 SIA sites in Choma and Ndola Districts. A questionnaire was administered to the caregiver to collect the childs demographic information and history of routine measles-rubella (MR) vaccination, and dried blood spot specimens were collected from the child. Specimens were tested for Immunoglobulin-G antibodies to measles and rubella viruses by enzyme immunoassay. 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% (118/2364) of children would not have otherwise received routine MR dose 1 without the SIA and 23.4% (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 of children were measles seropositive and 90% were 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 already have received routine vaccinations, the SIA reached a considerable number of susceptible children. Monitoring SIA effectiveness and efficiency is important to understand the benefits of vaccine delivery strategies in reaching un- and under-vaccinated children and may guide alternative strategies.