Measles outbreak investigation in Terego District, Uganda, February ─June, 2024

On April 30, 2024, Uganda’s Ministry of Health was notified of a measles outbreak in Terego District, following laboratory confirmation of measles virus in four of seven tested specimens. We investigated to assess the outbreak’s magnitude, identify transmission risk factors, and estimate measles vaccine coverage (VC) and effectiveness, to inform control measures. We defined a suspected case of measles as onset of fever and maculopapular rash, plus ≥ 1 of cough, coryza or conjunctivitis in a resident of Terego District from February 1, 2024 to June 1, 2024. A confirmed case was a suspected case with a positive measles-specific IgM antibody test. We used medical records and active case search in communities to find cases. We conducted a 1:1 unmatched case-control study to identify risk factors, and used multivariable logistic regression to estimate adjusted odds ratios (aORs). We estimated vaccine coverage using the percentage of vaccinated persons among the controls. We calculated vaccine effectiveness (VE) using formula VE = 1-MHaOR, where MHaOR is the Mantel-Haenzel (pooled) adjusted odds ratio for the association between measles infection and receiving at least a single dose of measles containing vaccine. We identified 136 measles case-patients, including 132 suspected and 4 confirmed cases, with no reported deaths. The overall attack rate (AR) was 53 per 100,000 population, with the highest AR observed among infants aged 0–6 months (132/100,000). Measles vaccination was found to be 73% protective against disease (adjusted odds ratio [aOR] = 0.27; 95% CI 0.10–0.70). Factors significantly associated with increased risk of measles included visiting a health facility during the exposure period (aOR = 3.8; 95% CI 1.3–11.4), having comorbidities (aOR = 2.4; 95% CI 1.01–5.7) and attending public gatherings (aOR = 6.1; 95% CI 2.6–15.0). Overall, VE was estimated at 73% and VC at 71%. The measles outbreak in Terego District was facilitated by suboptimal vaccine effectiveness, low vaccine coverage, nosocomial spread in health facilities, comorbidities, and community gatherings. Infants were the most affected. Strengthening routine immunization—particularly by extending coverage to infants < 9 months alongside timely isolation of cases and risk communication could mitigate measles transmission and prevent future similar outbreaks. Understanding and addressing the factors contributing to low vaccine effectiveness should be explored by the MoH.