Investigation of a cross-border measles outbreak in Moroto District, northeastern Uganda, March–September, 2024

Background On June 26, 2024, Ministry of Health was notified of a measles outbreak in Moroto District, northeastern Uganda, which had already resulted in four deaths. We investigated to assess magnitude of the outbreak, identify its source and risk factors, and recommend evidence-based control and prevention measures. Methods We defined a suspected case as onset of fever and maculopapular rash, and ≥ 1 of cough, coryza, or conjunctivitis in a resident of Moroto District from March 15 to September 22, 2024. A confirmed case was a suspected case with a positive measles-specific IgM test. We identified cases using health facility records and active case search within the health facilities. We conducted a hospital-based case-control study with a 1:2 ratio. Controls were children admitted to the pediatric ward with an alternative diagnosis but no history of fever, rash, or conjunctivitis. We used logistic regression to identify risk factors. We estimated the vaccine effectiveness (VE) from adjusted odds ratio (aOR) associated with vaccination (VE = 1-aOR) % and vaccination coverage (VC) from percentage of vaccinated controls. Results We identified 290 case-patients (275 suspected and 15 confirmed), with 6 deaths (case fatality rate [CFR] = 2%). The index case-patient was a 10-month-old child who had returned from the Western Turkana Region of Kenya, where a measles outbreak was ongoing. The overall attack rate (AR) was 19/10,000. Lotisan Sub-county (AR = 50/10,000) and children < 1 year (AR = 194/10,000) were the most affected. Being vaccinated was protective (aOR = 0.11, 95%CI = 0.03–0.4). Visiting a health facility 7–21 days before the onset of rash (aOR = 3.8, 95%CI = 1.3–11) and having malnutrition (aOR = 6.1, 95%CI = 1.3–27) increased the odds of contracting measles. Estimated VC was 67% and VE was 89% (95%CI = 60–97%). Conclusion The outbreak was likely imported from Kenya, and was propagated by low vaccination coverage, and healthcare visits within the exposure period. To prevent similar future outbreaks, it is crucial to improve vaccination coverage, strengthen triage systems at health facilities for prompt case identification and management, and enhance cross-border surveillance.