Background. In response to a hyperendemic situation involving invasive meningococcal C infections, a study was conducted using aggregated data from the National Inter-Regime Health Insurance Information System (Sniir-AM) to test the hypothesis of inadequate vaccination coverage (VC) against meningococcal C in Finistère among 1- to 24-year-olds targeted by the vaccination schedule recommendations. Materials and methods. The number of doses of conjugate vaccines reimbursed in 2010 and 2011, aggregated by age group (1–4 years, 5–9 years, 10–14 years, 15–19 years, 20–24 years), was used and compared to INSEE population estimates for 2009. Results. Vaccination coverage among 1- to 24-year-olds was 20.1% in Finistère, 18.5% in Brittany, and 19.4% in mainland France excluding Finistère. The rate in Finistère was 5% lower (42.2% vs. 47.3%, p<0.001) than the national rate excluding Brittany among 1- to 4-year-olds, and slightly higher in the other age groups. Discussion and conclusion. Vaccination coverage was insufficient to achieve herd immunity benefiting unvaccinated age groups. However, it does not reflect a particularly high-risk situation in Finistère. Aggregated data from Sniir-AM enabled rapid estimation of vaccination coverage in this alert context. The results obtained were consistent with those estimated from the General Practitioner Sample of Beneficiaries (EGB) of the General Health Insurance Scheme at the national level. They contributed to the High Council for Public Health’s urgent development of an advisory opinion aimed at strengthening vaccination for 1- to 24-year-olds in Finistère, promoting it in the rest of France, and proposing transitional vaccination for 2- to 11-month-olds in Finistère. (R.A.)