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).