Effectiveness of incidence thresholds for detection and control of meningococcal meningitis epidemics in northern Togo

Early outbreak detection is necessary for control of meningococcal meningitis epidemics. A weekly incidence of 15 cases per 100000 inhabitants averaged over 2 consecutive weeks is recommended by the WHO for detection of meningitis epidemics in Africa. This and other thresholds are tested for ability to predict outbreaks and timeliness for control measures. Meningitis cases recorded for 1990-97 in health centers of northern Togo were reviewed. Weekly and annual incidences were determined for each district. Ability of different weekly incidence thresholds to detect outbreaks was assessed according to sensitivity specificity and positive and negative predictive values. The number of cases potentially prevented by reactive vaccination in 1997 was calculated for each threshold. Outbreaks occurred in 1995-96 and in 1996-97. The WHO-recommended threshold had good specificity but low sensitivity. Thresholds of 10 and 7 cases per 100000 inhabitants in 1 week had sensitivity and specificity of 100% and increased the time available for intervention by more than 1 or 2 weeks respectively. A maximum of 65% of cases could have been prevented during 1997 epidemic with up to 8% fewer cases prevented for each week of delay in achieving vaccine coverage. In northern Togo thresholds of 7 or 10 cases per 100000 inhabitants per week were excellent predictors of meningitis epidemics and allowed more time for a reactive vaccination strategy than current recommendations. (authors)