Increasing incidence of pertussis before scheduled primary school booster vaccinations in Norway, 1998–2019

OBJECTIVE: In Norway, infant pertussis vaccination is scheduled by birth date, whereas primary school and adolescent boosters are administered to entire school classes during the 2nd and 10th school years, respectively. The interval between infant vaccination and primary school booster can thus vary from 5.5 to 7.5 years. To examine the timing of the primary school booster, we estimated pertussis incidence among children aged 2 to18 years. METHODS: We conducted a nationwide, registry-based retrospective cohort study of children born between January 1, 1998, and December 31, 2013, who had completed infant pertussis vaccination by the age of two years. We calculated incidence rates (IR) by school year and stratified by birth period (spring or autumn). We compared the average number of reported pertussis cases during the first two school years (ages 5.5-8.5) across the current schedule and two hypothetical scenarios: vaccination prior to 2nd school year or prior to school entry. RESULTS: Of 782,875 children eligible for the primary school booster 93% were vaccinated by the end of the 2nd school year. Pertussis incidence rose after school entry, peaking around 15 reported cases per 10,000 children annually, then declined to 5 following booster uptake. We estimated that advancing the primary school booster prior to the 2nd or 1st school year would reduce incidence by roughly 25% and 62%, respectively. Spring-born children had higher incidence rates than autumn-born peers, reflecting longer average intervals between infant and primary booster dose. CONCLUSION: The current timing of the primary school pertussis booster appears suboptimal. Advancing the booster prior to the first school year could significantly reduce the burden of pertussis during first years of primary school.