Effectiveness of Measles-Mumps-Rubella Vaccination Among Persons Residing in a Congregate Migrant Shelter During a Measles Outbreak: An Observational Cohort Study

Background On 7 March 2024, the Chicago Department of Public Health was notified of a positive measles test in a resident of a shelter housing >2100 persons. We estimated the effectiveness of measles-mumps-rubella vaccine (MMR) administered during the outbreak to guide its use as an acute intervention. Methods We used Pearson χ2 tests to compare unadjusted attack rates (ARs) by age group, number of days exposed, and sex. We estimated adjusted hazard ratios (aHRs) using time-dependent Cox proportional hazards regression models to assess the association between time-varying MMR dose status and the outcome of laboratory-confirmed measles; the final model was adjusted for time-varying exposure to measles virus and age group. We estimated risk-adjusted vaccine effectiveness (VE) as VE = (1 – aHR) × 100. Results Measles was diagnosed in 52 of 2243 (2.3%) residents who resided in the shelter for ≥1 day during 22 February–4 April 2024. ARs were highest among children aged <6 months (57.1%) and 6–11 months (55.6%) and among residents who were exposed for 8–13 days (10.1%). ARs did not differ by sex. Estimated 1-dose VE was 70% (95% confidence interval [CI], 28%–88%) and estimated ≥2-dose VE was 72% (95% CI, 21%–90%). Conclusions This study provided a unique opportunity to evaluate MMR VE in a contemporary outbreak setting with a high force of infection and could guide future responses in similar congregate settings. MMR was effective at preventing measles among shelter residents. Public health efforts that prioritize vaccinating persons without ≥2 MMR doses remain critical for limiting transmission.