Use of an observational cohort study to estimate the effectiveness of the New Zealand group B meningococcal vaccine in children aged under 5 years

Background In July 2004 a strain-specific vaccine was introduced to combat an epidemic of group B meningococcal disease in New Zealand. We estimated the effectiveness of this vaccine in pre-school-aged children. Methods We conducted a cohort analysis of all children in New Zealand who were aged 6 months to 5 years at the time the vaccine became available for that age group in their area. We defined cases as children who were diagnosed with laboratory-confirmed epidemic strain meningococcal disease. We calculated person-days-at-risk using data from the National Immunization Register and population estimates from Statistics New Zealand. We estimated vaccine effectiveness as 1 relative risk. Results Compared with unvaccinated children, fully vaccinated children were five to six times less likely to contract epidemic strain meningococcal disease in the 24 months after they became eligible to receive a full vaccination series, corresponding to an estimated vaccine effectiveness of 80.0 (95 confidence interval: 52.591.6) for children aged 6 months to 5 years and 84.8 (95 confidence interval: 59.494.3) for children aged 6 months to 3 years. Conclusions With over 3 million doses administered to individuals aged under 20 years throughout New Zealand, combined evidence from the Phase I and II clinical trials, the descriptive epidemiology of meningococcal disease, and this study provide evidence supporting the effectiveness of this vaccine in the 2 years following vaccination.