Effectiveness of serogroup C meningococcal polysaccharide vaccine:: Results from a case-control study in Quebec

Background. After a mass-immunization campaign in the province of Quebec, Canada, from 1992 to 1993, a case-control study was conducted to evaluate the effectiveness of the polysaccharide vaccine, while controlling for the potential confounding effects of selected risk factors for serogroup C meningococcal disease. Methods. The case patient group comprised 74 individuals with confirmed serogroup C meningococcal disease reported after the beginning of the campaign until 31 March 1998. Four control subjects, matched for age and place of residence, were randomly selected from the Quebec health insurance registry. Information on case patients was obtained from regional public health departments. Case patients and control subjects (or a family member) were interviewed by telephone. The analyses were conducted by using conditional logistic regression models. Results. Although the 95% confidence intervals (CIs) were large as a result of the small sample sizes, a high level of protection was found among children aged >= 6 years, during the first 2 years after vaccination (vaccine effectiveness, 95%; 95% CI, 68%-99%;), and protection remained high during the following 3 years (77%; P<.002 95% CI, - 364% to 99%;). For children aged 2-5 years, the estimated effectiveness was positive during the first 2 years (62%; 95% CI, -403% to 97%;) but was negative during the following period (-74%; 95% CI, -1956% to 85%;). Among children aged <2 years, there was no evidence of protection. Household crowding and disadvantaged socioeconomic conditions were associated with increased risk of disease. Conclusions. The polysaccharide vaccine remains a cost-effective option for the short-term protection of school-aged children and adults; however, conjugate vaccines are needed for younger children.