Introduction Measles is a vaccine-preventable viral infection of humans, primarily affecting children <5 years. During early 2019, outbreak of measles occurred in Ginnir district of Bale zone, Southeast Ethiopia. We investigated to describe the outbreak and identify risk factors. Methods We conducted a descriptive and 1:2 unmatched case-control study in Ginnir district from March 18 to April 29, 2019. Fifty-six cases and 112 controls were recruited. For descriptive study, we identified 1043 cases recorded on the line-list and for case-control study, cases were identified using national standard case-definition. Mothers of case-patients and controls were interviewed using structured questionnaire. We estimated vaccine efficacy (VE) from case-control study. We conducted bivariate and multivariable logistic regression. Results In four-months period, a total of 1,043 suspected measles cases epidemiologically linked to five laboratory confirmed cases reported. Of which, 555 (53.2%) were males and 714 (68.5%) were <5 years. The median age of cases was 36 months (IQR=12-60 months). The overall attack rate (AR) was 63/10,000 population with case fatality ratio of 0.5% (5 deaths/1043). Infant <9 months were the most affected age groups (AR=31/1000). Majority (79%) of measles cases were not vaccinated against measles. Last-year (2017/18) administrative measle vaccine coverage of the district was 76.7%. Being unvaccinated against measles (AOR=5.4, 95%CI=2.2-13.4), travel history (AOR=4.02, 95%CI=1.2-13.6), contact with measles case-patient (AOR=5.6, 95%CI=2.12-14.4) and mothers knowledge of measles transmission (AOR=0.36, 95%CI=0.15-0.87) were associated with measles infection. VE in children aged 9-59 months was 90% (95%CI=69-97%). Conclusion This confirmed measles outbreak was caused by failure to vaccinate, as indicated by the high VE, low administrative coverage, and 79% unvaccinated cases. Strengthening routine and supplementary immunization are required.

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