Pertussis epidemiology including direct and indirect effects of the childhood pertussis booster vaccinations, Norway, 1998-2019

Abstract

Background: The acellular pertussis vaccine has been used in the Norwegian national immunisation program since 1998. Following an increase in pertussis incidence in all age groups, booster doses were introduced for 7-8-year-olds in 2006, and for 15-16-year-olds in 2013. We assessed the effects of the booster doses on pertussis incidence in different age groups to inform potential changes in vaccination policy.

Methods: We included all pertussis cases notified to the Norwegian Surveillance System for Communicable Diseases in 1998-2019. We calculated annual incidence rates (IR, per 100,000 inhabitants) by age group. We estimated average annual changes in IRs (incidence rate ratios, IRR) for each age group for 2006-2012 and 2013-2019 using Poisson regression.

Results: In 1998-2019, 74,675 cases of pertussis were notified. Coinciding with booster introduction, between 2006 and 2012 the IR decreased among 8-15-year-olds (from 433 to 199/100,000, IRR 0.89 [95% confidence interval 0.88-0.90]). A similar decrease was seen between 2013 and 2019 among 16-19-year-olds (from 171 to 77/100,000, IRR 0.84 [0.82-0.86]). There was no significant change in IRs among children < 1 year of age between 2006 and 2012 (IRR 0.99 [0.95-1.04]) or 2013-2019 (IRR 0.96 [0.91-1.02]). The IR decreased in both periods among adults aged 20-39 and 40+ (IRR 0.94 [0.93-0.95] and 0.92 [0.91-0.92] in 2006-2012; IRR 0.97 [0.96-0.99] and 0.97 [0.96-0.99] in 2013-2019, respectively). Despite steady, high vaccination coverage, in 2013-2019, there was an increase in the IR among children aged 1-7 (63 to 86/100,000, IRR 1.05 [1.03-1.07]) and 8-15 years (88 to 122/100,000, IRR 1.08 [1.06-1.10]).

Conclusions: Pertussis booster doses have offered direct protection in the targeted age groups. Our findings suggest indirect protection in adults, while the incidence in infants hasn't changed. The recent increase in IRs among 1-15-year-olds warrants close monitoring and further evaluation of the vaccination schedule.

Keywords: Adolescent booster; Epidemiology; Incidence; Pertussis; School-age booster; Vaccination.