Investigation of a measles outbreak in a highly vaccinated middle school, France, 2023

In September 2023, a measles outbreak occurred in a middle school (Rhône valley, France), with the index case returning from Asia. Investigations involved case validation, virological analyses, contact tracing and checking vaccination records to determine measles vaccination coverage (VC) and attack rates (AR). Among 643 students, 49 measles cases occurred between 6 September and 18 October 2023 (AR = 7.6%). Two-dose vaccination coverage was 93.5% (601/643). Virological analyses confirmed the measles strains' clonality (genotype D8) and the imported origin. Concordance between health record vaccination status and immunological profile was established for 27 cases. In a sub-cohort of children (all cases and 309 non-cases), AR was 100% in unvaccinated children, and 43.7%, 16.5% and 3.2% among two-dose vaccinated children with the first dose administered at 6-8, 9-11 and ≥ 12 months, respectively. After multivariate binomial regression, vaccine effectiveness (VE) was 96.4% (95% confidence interval (CI): 91.4-98.5) after two-dose vaccination with the first dose at ≥ 12 months, confirming long-term effectiveness of measles-mumps-rubella vaccines. When the first dose was given at 9-11 and 6-8 months, respectively, VE was 83.3% (95% CI: 74.3-89.2) and 60.7% (95% CI: 10.6-82.7). This measles epidemic mainly affected unvaccinated or two-dose vaccinated children with first dose administered before age 12 months.