Pertussis vaccine effectiveness in a frequency matched population-based case-control Canadian Immunization Research Network study in Ontario, Canada 2009–2015

•Immunization provides highly effective protection from pertussis.•Children are most protected from pertussis in the first few years after immunization.•Protection is sustained for the first decade after the last dose of vaccine, but subsequently falls rapidly.•Method of control selection affects estimates of vaccine effectiveness. Resurgences of pertussis have occurred in several high-income countries, often linked to waning of immunity from acellular pertussis vaccines. The degree of waning observed has varied by study design and setting. In Ontario, pertussis has not shown a substantial resurgence in the past decade. The routine immunization schedule comprises three priming doses in infancy, toddler and pre-school doses, and an adolescent dose at 14–16 years of age. We estimated pertussis vaccine effectiveness (VE) through a case-control study of 1335 cases statutorily reported to public health in Ontario and occurring between January 1, 2009 and March 31, 2015, compared with 5340 randomly selected population controls, frequency-matched by age, primary-care provider and year of diagnosis. Pertussis cases met provincial confirmed or probable case definitions. We used multivariable logistic regression to estimate crude and adjusted odds ratios (aOR). VE against pertussis was sustained between 92% (95% confidence interval (95%CI) 88–95%) in 2–3 year olds and 90% (95%CI: 80–95%) in 8–9 year olds, but fell rapidly to 49% (95%CI: 2–73%) in children 12–13 years of age. VE following the teenage booster given at 14–16 years in Ontario reached 76% (95%CI: 52–88%) in 14–16 year olds and 78% (95%CI: −31 to 96%) in those 16–22 years old. For children who were up-to-date with the immunization schedule, VE declined from 87% (95%CI: 84–90%) during the first year to 74% (95%CI: 63–82%) after 8 or more years following their last dose of immunization. VE is high during the first decade of life but then falls rapidly. Protection is not fully restored by the teenage booster. Our findings are consistent with the localized outbreaks we observe in high school children and underline the importance of additional policies to protect infants.