Effectiveness of a mass immunization campaign against serogroup C meningococcal disease in Quebec.

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Abstract
A gradual increase in the number of cases of meningococcal disease (MCD) reported to the public health authorities was observed in Quebec in the late 1980s.1,2 This was associated with an increased proportion of cases caused by a virulent clone of serogroup C serotype 2a Neisseria meningitidis, an increased proportion of cases in teenagers and young adults, a high case fatality rate, and a high sequelae rate.3,4 In an attempt to control this outbreak, local immunization programs directed at school-aged children and adolescents were initiated in late 1991 and extended in 1992. By the autumn of that year, approximately 300 000 doses of polysaccharide vaccine had been administered, but the incidence of serogroup C MCD continued to stay high in the groups that were not vaccinated, and clusters appeared in previously unaffected areas. This generated enormous anxiety in the population, fueled by the media. As a result, local authorities decided to conduct a mass immunization program and to offer the vaccine free to all 1.9 million people living in the province between the ages of 6 months and 20 years. The campaign started in December 1992 and was completed by the end of March 1993.5 In total, approximately 1.6 million doses of vaccine were distributed, including the tetravalent (A, C, Y, W135) from Connaught, North New York, Ontario (24% of total doses), the bivalent (A, C) from SmithKline-Beecham, Rixensart, Belgium (4%), and the bivalent (A, C) from Mérieux, Lyon, France (72%). This intervention provided a unique opportunity to assess the effect of mass immunization on the epidemiology of MCD and to assess the medium-term effectiveness of the polysaccharide vaccine according to age at vaccination. This is especially important because immunization is increasingly used for controlling outbreaks or epidemics in different parts of the world6-9 and because of the controversy surrounding the induction of anergy to meningococcal C polysaccharide.10,11