Despite childhood vaccination coverage rates exceeding 75%, pertussis is still frequently reported in Canada. In Alberta, pertussis incidence ranged from 1.8 to 20.5 cases per 100,000 persons for 2004-2015. Most cases occurred in those aged < 15 years. We investigated pertussis vaccine effectiveness (VE) using a test-negative designed (TND) study.
All individuals who had undergone a real-time PCR laboratory test for Bordetella pertussis between January 1st 2004 and August 31st 2015, in the province of Alberta, Canada were included. Vaccination history was obtained from Albertas immunization repository. Vaccination status was classified as complete, incomplete, or not vaccinated, based on the provinces vaccination schedule. Multivariable logistic regression models were used to estimate adjusted odds ratios (aOR) and 95% confidence intervals (95% CI) for pertussis infection by time since last vaccination, comparing those with complete or incomplete vaccination to those not vaccinated. We adjusted for age, sex, income, urban/rural status, and the presence of a co-morbid condition. Vaccine effectiveness (VE) was calculated as [(1-aOR)*100].
Of 28,154 individuals tested, 2,297 (12.3%) tested positive for B. pertussis. Among those with complete vs. no vaccination, VE was 88% (95% CI 85-91%) at 1 year, 83% (95% CI 79-86%) at 1 to 3 years, 70% (95% CI 63-76%) at 4 to 6 years, 28% (95% CI 12-42%) at 7 to 9 years, and -4% (95% CI -53 to 29%) at 10 or more years since a last dose of a pertussis vaccine (Figure 1). VE was similar but attenuated in the incompletely vaccinated group, with a comparable waning of immunity.
Pertussis VE was high in the first year after vaccination, then declined noticeably after 5 years. Our results suggest there is a large number of adolescents and adults susceptible to pertussis. Regular boosters throughout childhood, adolescence, and during pregnancy are critical to protect those at greatest risk of infection and complications. Further validation of the strengths and weaknesses of the TND for assessing pertussis VE is needed.
Figure 1. Pertussis VE by year since last vaccination in those with complete vaccination status.
C. A. Bell,
K. A. Simmonds, None
L. W. Svenson, None
N. S. Crowcroft, None
K. Schwartz, None
J. C. Kwong, None
S. Mahmud, None
M. L. Russell, Novartis Pharmaceuticals Canada Inc.: Grant Investigator and Unconditional Research Grant , Grant recipient . Merck Frosst Canada Inc.: Grant Investigator and Unconditional Research Grant , Grant recipient .