Introduction

There has been a global resurgence in pertussis in recent years, with the highest rates of hospitalization and death in infants too young to be fully protected by routine childhood vaccination series.1-3 Infection of young infants occurs primarily through household contacts, with new mothers identified as the most common sources of transmission.4 Pertussis vaccination during pregnancy protects the mother from pertussis infection but more importantly offers passive immunity to her newborn via transplacental transport of maternal antibodies and is currently considered the most successful and effective intervention to prevent early infant disease.5 Maternal pertussis vaccination at least seven days before delivery has been shown to protect against pertussis in up to 91% of infants in the first two months of life.6, 7 Following the implementation of government funded maternal pertussis vaccination programs for pregnant women in different countries there has been an increasing general pertussis vaccine uptake in pregnancy.8 However uptake varies across and within countries in pregnant women and is suboptimal.8 One of the reasons for low pertussis vaccination uptake is healthcare providers’ and/or women’s concerns about the safety of the vaccine during pregnancy.9, 10
Several systematic reviews11-14 have demonstrated that maternal pertussis vaccination during pregnancy does not adversely affect obstetric or neonatal outcomes, although the quality of the evidence in underlying studies is low. Many previous studies have investigated the safety of pertussis vaccination during pregnancy retrospectively, due to the relatively cheaper cost, fewer ethical concerns, and difficulty in recruiting pregnant women to randomized controlled trials. Three retrospective cohort studies15-17 reported a small but statistically significant increased relative risk of chorioamnionitis in women who had received pertussis vaccination during pregnancy. This unreplicated finding merits further investigation since chorioamnionitis is associated with severe short-term and long-term neonatal complications.18Most observational studies have been unable to account for the time-dependent nature of exposure to pertussis vaccination during pregnancy. To accurately determine any risk of maternal pertussis vaccination for pregnant women and their infants, prospectively designed studies incorporating statistical approaches suitable for analysing time-dependent associations between maternal pertussis vaccine exposure on pregnancy and birth outcomes are warranted. The primary aim was to prospectively assess pregnancy and birth outcomes following pertussis vaccination during pregnancy considering time‐dependent vaccine exposure and using the most comprehensive set of potential confounding variables considered to date.