The 1990 meningococcal meningitis epidemic of Sarh (Chad): how useful was an earlier mass vaccination?

A large outbreak of meningococcal meningitis (Serogroup A) occurred in southern Chad in 1990. We describe the epidemic in the town of Sarh, where a mass vaccination against meningococcal meningitis had taken place two years before, in 1988 (estimated coverage: 66%). Early warning that an epidemic was imminent was given at the end of February, following more than 15 recorded cases per 100,000 population on 3 consecutive weeks. This threshold proved to be adequate to predict the outbreak. A total of 721 cases were recorded at Sarh hospital and at a nearby health centre. Direct agglutination tests confirmed that Neisseria meningitis serogroup A was the only causative agent. The overall incidence rate in this population of 80,000 was 0.9%. The highest weekly incidence rate was 186 cases per 100,000 population. The mean age of the patients was 12.6 years and age-specific incidence rates ranged from 0.23% to 1.42%. The male/female sex ratio was 1.36. Overall, the mortality among hospitalized patients was 7.9%. Mortality increased during the epidemic. The major risk factor for dying was the delay until reaching hospital. Only 4 out of 29 interviewed parents said that their child had been vaccinated two years before. Among adult patients this proportion was 12/38 (32%). Because of the small numbers and because of the impossibility to check the vaccination records it was not possible to assess precisely the impact of earlier mass vaccination. However, the previous mass vaccination did not prevent this major epidemic and its impact is likely to have been unimportant.