Impact of a mass immunization campaign against serogroup C meningococcus in the Province of Quebec, Canada.

A mass immunization campaign was conducted in the Province of Quebec, Canada, during the winter of 1993, following an increase in the incidence of meningococcal disease, which was mainly caused by a virulent clone of Neisseria meningitidis, serogroup C, serotype 2a. About 1.6 million doses of the polysaccharide vaccine were administered, covering 84% of the target population aged between 6 months and 20 years; the overall cost was about 25.5 million Canadian dollars. Cases notified to the regional health authorities by clinicians, hospital laboratories, and the provincial reference laboratory from January 1990 up to March 1994 have been included in the analysis. In the first year following the campaign, the incidence of the disease dropped markedly among vaccinees as well as the unvaccinated fraction of the target population, while it remained unchanged among persons aged more than 20 years. This suggests the existence of herd immunity. The overall field efficacy of the vaccine was 79%, more in teenagers and less in under-5-year-olds. A minimum of 37 cases were prevented during the first year.