Vaccine effectiveness of measles vaccine and associated factors under real-life conditions in children aged 9-59 months in a context of epidemic resurgence in the Democratic Republic of the Congo: An unmatched case-control study with Test-Negative Approach

Background Measles remains a major public health problem in the Democratic Republic of Congo (DRC), despite vaccination efforts. In November 2023, the DRC totaled 298,132 cases of measles, with a case-fatality rate of 1.92%. Children aged ˂5 represented 73%, of whom 31% were vaccinated, 27% unvaccinated and 42% with unknown vaccination status. The aim of this study was to estimate the actual efficacy of the measles vaccine (VAR) in a context of epidemic resurgence. Methods An unmatched case-control study was conducted in 25 provinces of the DRC from January 2020 to December 2023. It included 4,317 serologically confirmed cases and 3,732 serologically negative controls. The national database of the laboratory-confirmed measles epidemiological surveillance system was used. The "test-negative" approach was used. Logistic regression analysis was adjusted by a propensity score based on vaccination status. This score enabled us to reduce confounding factors and correctly estimate the adjusted vaccine effectiveness of the VAR in DRC. Results Among the children included in the study, only 17.0% (CI95%: 14.3–19.7) of children who received at least 1 dose of VAR developed measles, compared with 83.0% (CI95%: 81.7–84.2) of non-vaccinated children. Adjusted VAR vaccine effectiveness was 35.4% (CI95%: 34.3–36.4). Vaccinated children had a reduced probability of infection compared with non-vaccinated children, with an adjusted OR of 0.65 (CI95%: 0.75–0.87). Age, year of diagnosis and province were the factors influencing the adjusted OR. Conclusions MV shows partial but significant effectiveness in reducing the risk of measles. The resurgence of measles epidemics in the DRC and the lower-than-expected MV vaccine mean that the quality of routine immunization strategies and Supplementary Immunization Activities must be improved. The systematic introduction of the second dose of MV into the routine EPI already underway is crucial. Regular evaluations of the VE of MV will enable us to modulate strategies towards the elimination of this disease by 2030.