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

BACKGROUND: Early outbreak detection is necessary for control of meningococcal meningitis epidemics. A weekly incidence of 15 cases per 100 000 inhabitants averaged over 2 consecutive weeks is recommended by the World Health Organization (WHO) for detection of meningitis epidemics in Africa. This and other thresholds are tested for ability to predict outbreaks and timeliness for control measures. METHODS: Meningitis cases recorded for 1990-1997 in health centres 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. RESULTS: Outbreaks occurred in 1995-1996 and in 1996-1997. The WHO-recommended threshold had good specificity but low sensitivity. Thresholds of 10 and 7 cases per 100,000 inhabitants in one week had sensitivity and specificity of 100% and increased the time available for intervention by more than one or two weeks, respectively. A maximum of 65% of cases could have been prevented during the 1997 epidemic, with up to 8% fewer cases prevented for each week of delay in achieving vaccine coverage. CONCLUSIONS: In northern Togo, thresholds of 7 or 10 cases per 100,000 inhabitants per week were excellent predictors of meningitis epidemics and allowed more time for a reactive vaccination strategy than current recommendations.

[An epidemic of meningococcal meningitis in the region of Savanes in Togo in 1997: research and control strategies]. / Une épidémie de méningite à méningocoque dans la région des Savanes au Togo en 1997: investigation et stratégies de contrôle.

Neisseria meningitidis is responsible for high levels of morbidity and mortality in the developing countries of the African meningitis belt. There are frequent meningococcal meningitis epidemics in this region affecting almost 1,000 people in every 100,000 (1%). Epidemics generally occur during the dry season but the interval between epidemics is variable (between 2 and 25 years). The reasons for these recurrent epidemics are unclear. There is a safe and effective polysaccharide vaccine against meningococci A and C. Unfortunately, the immunity it provides decreases with time, especially in young children (aged less than 5 years) and it is thus not included in the Expanded Program on Immunization (EPI). WHO recommends mass vaccination using a threshold approach. This control strategy is effective if vaccination begins very soon after the threshold is crossed. There was an outbreak of group A meningococcal meningitis in the Savanes region of northern Togo in December 1996. The national surveillance system put out an alert and control measures were implemented. These involved improvement of the surveillance system, and containment immunization in villages for early cases followed by a mass immunization campaign in the entire region, distribution of oily chloramphenicol and decentralized case management. The target population for mass vaccination included everyone older than 6 months of age living in the Savanes region. The aim was to vaccinate at least 80% of the target population. There were 2,992 cases of meningitis reported in the Savanes region between December 1996 and May 1997 (in a population of about 500,000). This gives a cumulative incidence rate of 581 per 100,000 population. The epidemic was bimodal, with the first peak in the number of cases occurring at the end of January and the second peak in March. There were 60,700 vaccinations in two of the four districts of the region in December and January, as part of the containment strategy and 346,469 vaccinations in the four districts of the region during February, as part of the mass vaccination campaign. By the end of the mass campaign, 67.3% of the target population in the region as a whole had been vaccinated, with 61% vaccinated in the Kpendjal district and 78% in the Oti district. There was an increase in the number of cases 2 weeks after the end of the mass vaccination campaign. This was attributed to the inadequate level of vaccination achieved. Only 52% of the urban population of Dapaong were vaccinated. The national surveillance system put out an alert early in the epidemic. The intervention was planned and adapted according to the progression of the epidemic, and national and international efforts were well coordinated. This emphasizes the importance of a rapid reaction from the surveillance system and of the choice of strategy for dealing with meningitis epidemics in sub-Sahelian Africa.