Early effectiveness of the pertussis vaccine is followed by a rapid decline, especially in cases of 4 or more years since last vaccination, according to research published in CMAJ.

Kevin L. Schwartz, MD, from the Institute for Clinical Evaluative Sciences and the Institute of Health Policy, Management, and Evaluation at the University of Toronto, and colleagues used a test-negative design­—a type of nested case-control study—to assess the effectiveness of the pertussis vaccine. Vaccine effectiveness was calculated as (1-OR [odds ratio]) x 100. Study participants were residents of Ontario, born between April 1, 1992 and January 1, 2013. Vaccinations given within 14 days of testing were excluded, due to insufficient time to develop an immunologic response.

The researchers examined data from 5867 individuals (486 test-positive and 5381 test-negative controls). Of the 486 test-positive cases, 39.7% (n=193) had up-to-date pertussis vaccination; 27.6% (n=134) were partially vaccinated, and 32.7% (n=159) were unvaccinated. In comparison, 62.9% (n=3384) of the control subjects were up-to-date, 22.5% (n=1212) were partially vaccinated, and 14.6% (n=785) were unvaccinated. Among all subjects, 24.7% test-positive cases and 20.8% controls received 5 or more doses.

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Dr Schwartz and colleagues found that unadjusted vaccine effectiveness declined over time by 10% per year since last vaccination in subjects who were up-to-date or partially vaccinated. The number of positive cases proportionally increased by 2% for each year since last vaccination (P <.001). Adjusted estimates of vaccine effectiveness for up-to-date participants were 80% (95% confidence interval [CI] 71%-86%) at 15-364 days, 84% (95% CI 77%-89%) at 1-3 years, 62% (95% CI 42%-75%) at 4-7 years and 41% (95% CI 0%-66%) and 8 years or more since last vaccination. Additionally, the researchers observed waning immunity in the acellular vaccine (adjusted OR 1.27, 95% CI 1.20-1.34) as well as increased odds of pertussis (adjusted OR 2.15, 95% CI 1.30-3.57).

“We observed high effectiveness of the pertussis vaccine within 3 years of vaccination, but with clear evidence of waning immunity beyond 4 years and little-to-no protection beyond 7 years from last vaccination,” wrote Dr Schwartz and colleagues. “The odds of pertussis increased by 27% each year that passed after receipt of an acellular vaccine. Individuals primed with acellular vaccine had a 2.2 times higher odds of disease than those primed with the previously used whole-cell vaccine.”

Study limitations include a potential for false-negative results—misclassifying those cases as controls—due to the test-negative design and leading to an underestimation of vaccine effectiveness. The researchers also noted a potential for misclassification bias of vaccination status.

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1. Schwartz KL, Kwong JC, Deeks SL, et al. Effectiveness of pertussis vaccination and duration of immunity. CMAJ. 2016; doi:10.1503/cmaj.161048