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

Background Early outbreak detection is necessary for control of meningococc al meningitis epidemics. A weekly incidence of 15 cases per 100 000 inhabit ants averaged over 2 consecutive weeks is recommended by the World Health O rganization (WHO) for detection of meningitis epidemics in Africa. This and other thresholds are tested for ability to predict outbreaks and timelines s for control measures. Methods Meningitis cases recorded for 1990-1997 in health centres of northe rn Togo were reviewed. Weekly and annual incidences were determined for eac h district. Ability of different weekly incidence thresholds to detect outb reaks was assessed according to sensitivity, specificity, and positive and negative predictive values. The number of cases potentially prevented by re active vaccination in 1997 was calculated for each threshold. Results Outbreaks occurred in 1995-1996 and in 1996-1997. The WHO-recommend ed threshold had good specificity but low sensitivity. Thresholds of 10 and 7 cases per 100 000 inhabitants in one week had sensitivity and specificit y of 100% and increased the time available for intervention by more than on e or two weeks, respectively. A maximum of 65% of cases could have been pre vented during the 1997 epidemic, with up to 8% fewer cases prevented for ea ch week of delay in achieving vaccine coverage. Conclusions In northern Togo, thresholds of 7 or 10 cases per 100 000 inhab itants per week were excellent predictors of meningitis epidemics and allow ed more time for a reactive vaccination strategy than current recommendatio ns.