Timely detection of meningococcal meningitis epidemics in Africa

Background Epidemics of meningococcal disease in Africa are commonly detect ed too late to prevent many cases. We assessed weekly meningitis incidence as a tool to detect epidemics in time to implement mass vaccination. Methods Meningitis incidence for 41 subdistricts in Mali was determined fro m cases recorded in health centres (1989-98) and from surveillance data (19 96-98). For incidence thresholds of 5 to 20 cases per 100 000 inhabitants p er week, we calculated sensitivity and specificity for detecting epidemics, and determined the time lapse between threshold and epidemic peak. Findings We recorded 9084 meningitis cases. Clinic-based weekly incidence o f 5 and 10 cases per 100 000 inhabitants detected all meningitis epidemics (sensitivity 100%, 95% CI 93-100), with median threshold-to-peak time of 5 and 3 weeks. Under-reporting reduced sensitivity: only surveillance thresho lds of 5 or 7 cases per 100 000 inhabitants per week detected all epidemics . Crossing the lower threshold before the 10th calendar week doubled epidem ic risk relative to crossing it later (relative risk 2.1, 95% CI 1.4-3.2). At 10 cases per 100 000 inhabitants per week, specificity for outbreak pred iction was 88%, 95% CI 83-91). For populations under 30 000, 3 to 5 cases i n one or two weeks predicted epidemics with 85% to 97% specificity. Interpretation Low meningitis thresholds improve timely detection of epidem ics. Ten cases per 100 000 inhabitants per week in one area confirm epidemi c activity in a region, with few false alarms. An alert threshold of 5 case s per 100 000 inhabitants per week allows time to investigate, prepare for an epidemic, and initiate mass vaccination where appropriate. For populatio ns under 30 000, the alert threshold is two cases in a week. High quality s urveillance is essential.