Impact of previous immunisation on the incidence of meningococcal disease during an outbreak in a Sahelian area of Senegal.

The occurrence of an outbreak of meningitis during three consecutive years in a study area under demographic and epidemiologic longitudinal surveillance allowed evaluating the impact of mass immunisation campaigns on the incidence of meningitis. During an outbreak of meningitis in the neighbouring region occurred 2 years before the first epidemic wave in the study area, 8 out of the 30 villages of the zone were immunised. The incidences of meningitis in these villages were compared with those of the villages that did not benefited from mass campaign. It appeared a very significant difference between the two groups of villages. More than a half of the cases of meningitis seemed to be avoided in the vaccinated villages compared to the others, suggesting that a previous immunisation limits the diffusion of the epidemic. After the second outbreak hit the study zone, a mass immunisation campaign concerned all the 30 villages. The incidences of meningitis were significantly different between villages according to the observed vaccine coverage. These results indicate that preventive immunisation could have a significant impact on meningitis outbreak diffusion.