Measles outbreak investigation in Kakumiro District, Uganda, February-May 2024.

Background: On April 7, 2024, the Uganda Ministry of Health was notified of a measles outbreak in Kakumiro District involving death of a suspected case. We investigated to determine the scope of the outbreak, assess risk factors for disease transmission, and recommend evidenced-based interventions.

Methods: We defined a suspected case as onset of fever and maculopapular generalized rash with ≥ 1 of cough, coryza, or conjunctivitis in any resident of Kakumiro District during February-May 2024. A confirmed case was a suspected case with laboratory confirmation for measles Immunoglobulin M (IgM) antibody. We line-listed cases, performed descriptive analysis, and conducted a 1:1 case-control study with 100 randomly selected cases and village-matched controls. We identified risk factors using logistic regression and estimated vaccine coverage using the percentage of eligible controls who had received ≥ 1 dose of measles vaccine. We calculated Vaccine Effectiveness (VE) as VE = 1 - OR _adj  × 100%, where OR_adj is the adjusted odds ratio associated with having received ≥ 1 dose of measles vaccine.

Results: We identified 188 suspected cases, including 6 (3.2%) confirmed and 1 (0.5%) death. The overall attack rate (AR) was 67/100,000 persons. Children aged < 9 months (AR = 232/100,000) and those aged 9 months-≤ 5 years (AR = 177/100,000) were the most affected. The most affected sub-counties were Kisengwe (AR = 313/100,000), Kasambya (AR = 126/100,000) and Kakumiro Town Council (AR = 110/100,000). Non-vaccination (aOR = 2.9, 95%CI 1.1-7.6), exposure to a measles case in a health facility during exposure period (aOR = 47, 95%CI 6.09-369) and exposure to measles case in the same household during exposure period (aOR = 9.3, 95%CI 2.9-30) were associated with measles infections. Vaccine coverage was 88% (95%CI 79%-94%) and vaccine effectiveness was 65% (95%CI 13%-91%). We observed crowding and lack of triaging/isolation in health facilities.

Conclusions: This outbreak was facilitated by non-vaccination and propagated by exposure to infected persons in health facilities and households. We recommended to MoH to conduct a supplementary immunization activity that included children < 9 months in the target group. Triaging and isolation of cases might help to reduce the spread of measles in future outbreaks. There is also need to develop strategies to improve vaccine effectiveness in the district.