Results

Of 1364 pregnant women enrolled, 12 withdrew access to their medical records, six were lost to follow up, and 14 delivered elsewhere; all 32 women were excluded from our final analyses. After excluding 60 pregnancies that ended before 32 weeks’ gestation, our final cohort consisted of 1272 women (Figure 1). At enrolment, the mean (SD) maternal age of nulliparous women was 25.9 (4.9) years (range: 15–45 years). A total of 1040 (81.7%) women attended their first antenatal visit in first trimester at a mean gestational age of 11.4 (1.7) weeks.
The uptake of pertussis vaccination was 80.1%. Of the 1019 women who received maternal pertussis vaccination, 77.8% (n=790) received the pertussis vaccine within the recommended timeframe of 28-32 weeks, 2.7% (n=28) before 28 weeks (range 12-27 weeks) and 19.4% after 32 weeks’ gestation. The mean gestational age at vaccination was 30.3 (2.8) weeks. Both pertussis and influenza vaccinations were administered in 555 of 1272 (43.6%) pregnancies. Those women who did not receive maternal pertussis vaccination were more likely to have no previous history of termination and/or miscarriage, be younger, in the healthy weight range, smoke cigarettes, use illicit drugs, physically inactive, in the lowest household income group, have lower educational attainment and were less likely to take micronutrient supplements pre-conception or during pregnancy compared with vaccinated pregnant women. Women who received pertussis vaccination were more likely to receive influenza vaccine (Table 1).
Of the 1272 women, 82 had a PTB (6.4%). The mean gestational age at delivery was 39.4 (1.5) weeks. After adjusting for covariates, women who had received pertussis vaccination during pregnancy had on average 0.22 weeks (95% CI 0.001, 0.44) longer gestation at delivery than unvaccinated women (Table 2). The time-dependent Cox proportional hazards regression model shows that receiving pertussis vaccination during pregnancy did not increase the risk of PTB (aHR 0.99, 95% CI 0.47, 2.07), spontaneous PTB (aHR 0.99, 95% CI 0.57, 1.70) or PPROM (aHR 1.01, 95% CI 0.52, 1.97) (Table 2). Our time dependent analyses also indicated that there was no increased risk for maternal hypertensive disorders (i.e. GH, PE), or hospitalisation with acute respiratory/influenza-like illness among vaccinated women compared with unvaccinated women (Table 2). In log-binomial models, maternal pertussis vaccination was not associated with chorioamnionitis, placental abruption, or postpartum hemorrhage (Table 3).
Of all 1272 births included in this study, 1269 (99.7%) were live births and three (0.2%) were stillbirths at term. The majority of infants (93.5%, n= 1190) were born at term. The mean birthweight of the infants was 3368 grams. In the multivariable linear regression model, infants born to vaccinated mothers were on average 44.6 g heavier than infants born to unvaccinated mothers but the confidence intervals were wide (95% CI -26.0 g, 115.3 g) (Table 2). All birth outcomes had an adjusted relative risk of less than one, although all confidence intervals were wide (Table 2 & 3).
The time-dependent Cox proportional hazards regression models demonstrate that receiving pertussis vaccination during pregnancy was not associated with increased risk of delivering LBW infants (aHR 0.72, 95% CI 0.41, 1.27), LBW at term infants (aHR 0.67, 95% CI 0.29, 1.55) or SGA infants (aHR 0.80, 95% CI 0.53, 1.20) (Table 2). Our log-binomial models also suggest there was no increased risk of other adverse perinatal outcomes including Apgar scores <7 at one and five minutes, admission to the neonatal care unit, mechanical ventilation, and respiratory distress syndrome following pertussis vaccination during pregnancy (Table 3). Adjustment for influenza vaccination did not appreciably change any of the findings (Table S1 & S2).