Real-world effectiveness of measles vaccine and associated factors among children aged 9–59 months during epidemic resurgence in the Democratic Republic of Congo: a nationwide test-negative case-control study

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BACKGROUND: Measles remains a major public health concern in the Democratic Republic of Congo (DRC) despite ongoing vaccination efforts. During 2023, the country reported nearly 300,000 suspected cases, disproportionately affecting children under five years of age. This study aimed to estimate the real-world effectiveness of the measles vaccine (MV) and associated factors among children aged 9–59 months. METHODS: We conducted a nationwide test-negative case-control study using laboratory-based surveillance. Data was collected between January 2020 and December 2023 across 25 provinces. Children aged 9–59 months with documented vaccination status were included. Cases were 4,317 IgM-positive measles children, and controls were 3,732 IgM-negative children. Multivariable logistic regression was performed adjusted for age (months), sex, year of diagnosis, and province of residence. Vaccine effectiveness (VE) was calculated as (1-adjusted odds ratio x 100). RESULTS: Among the included children, vaccination was significantly associated with reduced odds of measles (aOR = 0.59; 95% CI 0.52–0.66; p˂0.001), corresponding to an adjusted vaccine effectiveness of 41.4% (95% CI: 34.4–47.8). Vaccination coverage in the analytic sample was 22.4%. Significant temporal and geographical heterogeneity were observed, particularly during the 2022–2023 epidemic resurgence. CONCLUSIONS: The moderate VE observed in nationwide surveillance data highlights operational gaps in routine immunization delivery that require targeted program strengthening.

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