Censoring Participants

Participants were followed up until they had pertussis diagnosed, died, moved to another county, or changed healthcare providers (“moved or gone elsewhere” in WA-IIS), or until the end of the follow-up period on 31 January 2017, whichever came first (Supplementary Material B). Because participants were not followed up over time as in a prospective cohort study, vaccination dates for pediatric vaccines administered between birth through 9 years of age were considered proxies for continued enrollment in WA-IIS and residence in King County. Participants are flagged as “inactive” in the registry if they moved out of state or died. However, this variable is inconsistently recorded. For our analysis, we assumed that a participant is “active” unless he or she is marked “inactive” or “died” and the date of becoming inactive is recorded.