Abstract
OBJECTIVE: To assess vaccine effectiveness through enhanced disease surveillance following the change in childhood immunization programs in 1995, when all provinces and territories chose to use polyribosyl ribitol phosphate-tetanus protein (PRP-T) Haemophilus influenzae type b (Hib) conjugate vaccine, generally in combination with diphtheria-pertussis-tetanus inactivated polio vaccine (DPT-IPV) (as PENTA vaccine) because the protective efficacy of this regimen had not been directly measured. DESIGN: Prospective, active, laboratory-based Hib case surveillance was implemented in British Columbia and Alberta, and enhanced, stimulated laboratory surveillance in Ontario during 1995 to 1997, centred on invasive infections in children. Case details and immunization histories were uniformly collected and centrally collated. MAIN RESULTS: Thirty-eight Hib cases were detected, but only 12 cases arose among PENTA-eligible children, an attack rate of 0.85 cases/100,000 child-years of observation. Annual case totals declined from 20 in 1995 to seven in 1997, when only one to three cases were encountered in each province and the incidence rate in children under age five years was 0.6/100,000. Only four cases occurred after primary immunization with PENTA, a failure rate of 0.28 cases/100,000 child-years of observation. Three cases among PENTA-eligible children reflected parental refusal of infant vaccinations, accounting for 25% of cases in eligible children. CONCLUSIONS: PRP-T conjugate vaccine was highly effective when given in combination with DPT-IPV vaccine. Provincial programs that used this regimen resulted in the near elimination of invasive Hib disease in children, but unimmunized children remain at risk.