Methods

The current study draws on data collected as part of a prospective cohort study (STOP), which aims to develop screening tests to identify adverse pregnancy outcomes. Healthy nulliparous pregnant women were recruited at two major maternity hospitals, the Lyell McEwin Hospital, the tertiary hospital serving a lower socio-economic community in Adelaide’s Northern suburbs and the Women’s and Children’s Hospital, the primary tertiary maternity hospital for complex care in metropolitan Adelaide, South Australia. Between March 2015 and December 2017, nulliparous women with a singleton pregnancy attending their first antenatal clinic between 9+0 and 16+0 weeks’ gestation were enrolled as part of the prospective cohort study described elsewhere.19, 20Women were excluded if they were considered already at high risk of pregnancy complications at screening (i.e. experienced three or more previous miscarriages or with pre-existing hypertension or diabetes). Participants were followed prospectively, with vaccination, pregnancy, and birth outcome data collected by research midwives. As pertussis vaccination was recommended to be administered between 28-32 weeks’ gestation, we restricted the analyses to data from women whose pregnancies reached at least 32 weeks’ gestation to allow all women to have had the opportunity to receive the pertussis vaccine.
The exposure of interest was pertussis-containing vaccine (tetanus-diphtheria-acellular pertussis: dTpa). A research midwife interviewed and collected maternal vaccination status of the women during their study visit interview at 32-36 weeks’ gestation. Maternal vaccination date and gestation at administration were recorded. Following delivery, a research midwife verified final vaccination status by reviewing Pregnancy-Hand-Held-Records and interviewing women. Pregnancy-Hand-Held-Records are the primary medical record of pregnancy care in South Australia.
Pregnancy outcomes assessed were gestational hypertension (GH), preeclampsia (PE), chorioamnionitis and/or funisitis, pre-delivery hospitalisation due to acute respiratory infections or influenza-like illness, premature rupture of membranes (PPROM), placental abruption, spontaneous preterm birth and preterm birth. Birth outcomes included small for gestational age (SGA), low birthweight (< 2500g) (LBW), LBW at term (≥ 37 weeks’ gestation), Apgar scores at 1 and 5 min, neonatal care unit admissions, respiratory distress and mechanical ventilation.
Pregnancy and birth complications were diagnosed using the Brighton Collaboration consensus list of terms, 21 and The Global Alignment of Immunization Safety Assessment in Pregnancy (GAIA) project.22 GH was defined as hypertension [systolic BP (SBP) ≥ 140 mmHg or diastolic BP (DBP) ≥ 90 mmHg] after 20 weeks of gestation in previously normotensive women. PE was defined as GH with proteinuria (24 h urinary protein ≥ 300 mg or spot urine protein: creatinine ratio ≥ 30 mg/mmol creatinine or urine dipstick protein ≥ 2+) or any multi-organ complication of PE, including SGA age infant. Suspected Chorioamnionitis was considered to be present only with a physician’s diagnosis, which was dependent on maternal fever ≥ 38 °C, with at least two of the following: maternal tachycardia, fetal tachycardia, uterine tenderness, foul odour of amniotic fluid, or maternal leucocytosis or increased CRP. Funisitis is a histopathologic diagnosis, and it is the extension of infection or inflammation to the umbilical cord. Preterm birth (PTB) was defined as any birth after 20+0 and before 37+0 weeks of gestation. SGA was defined as neonates with a birthweight < 10th percentile customized for maternal factors including maternal height, booking weight, ethnicity and gestational age at delivery.
During the first study visit at 9–16 weeks’ gestation, information was obtained regarding baseline socio-demographic, lifestyle and clinical characteristics including age, ethnicity, level of education, household income, employment, exercise, smoking, supplement use, intake of alcohol and recreational drugs, medical and obstetric history, and complications during the current pregnancy. Participants also completed questionnaires assessing stress levels in the past month (Perceived Stress Scale (PSS-10)) 23, current anxiety symptoms (short form of the Spielberger State–Trait Anxiety Inventory (STAI))24 and depressive symptoms during pregnancy (Edinburgh Postnatal Depression Scale (EPDS)). 25
Demographic, lifestyle and clinical characteristics of participants were summarized descriptively by pertussis vaccination exposure during pregnancy. Continuous variables were summarized as mean with standard deviation (SD) or median with interquartile range (IQR), as appropriate, while counts and percentages were used to summarize categorical variables. To assess if there was an association between maternal pertussis vaccination status and each of the outcome variables, we initially conducted independent samples t-tests and Mann-Whitney U tests, as appropriate, for continuous variables and chi-square tests of association for binary and categorical variables.
The timing for vaccination exposures and time-at-risk windows were calculated for each time-sensitive pregnancy and birth outcome, accounting for the temporal nature of each outcome of interest. For instance, women were at risk for PTB from 20+0 until 36+6 weeks of gestation but had to attain at least 32 weeks’ gestation for inclusion in the analysis data set. Cox proportional-hazards models with gestational age in weeks as the underlying time metric were used to derive hazard ratios (HRs) that compared the hazard rates between vaccinated and unvaccinated women for time-sensitive outcomes. Vaccination status was treated as a time-varying exposure in these models, in that each vaccinated woman’s pregnancy was divided into unvaccinated and vaccinated exposure periods. Thus, a woman who did not receive the vaccine during pregnancy was classified into the unvaccinated group in any risk set, whereas a woman who received the vaccine at some point during her pregnancy was initially classified as unvaccinated and then classified as vaccinated from the time at vaccination onwards.
We used log-binomial models to estimate risk ratios (RR) and adjusted risk ratios (aRR) comparing the risk of late onset or early postpartum adverse pregnancy outcomes and time-independent birth outcomes between vaccinated and unvaccinated mothers. A multivariable linear regression model was applied to compare the difference in mean gestational age at delivery and mean birthweight by maternal pertussis vaccination status. For all multivariable models, age, level of education, ethnicity, gravidity, annual household income, alcohol intake, recreational drug use, smoking, pre-pregnancy body mass index (BMI), use of micronutrient supplements, asthma, assisted reproductive treatment and current psychological states were amongst the variables selected as potential confounders based on evidence in the literature11-14guided by directed acyclic graphs. Sensitivity analyses were conducted in all multivariable models to evaluate whether the effects of maternal pertussis vaccination on pregnancy and birth outcomes were maintained after adjustment for receipt of maternal influenza vaccination. The overall missing covariate data at baseline was <5% and therefore all available data were used in the analyses of all pre-specified outcomes. For all analyses, p values < 0.05 were considered statistically significant. We did not correct for multiple comparisons to minimize the risk of Type II errors. Data were recorded in a REDCap 26, 27 online database and all statistical analyses were conducted using Stata version 15. Written informed consent was obtained from all women. Personal identifying information in the study database was eliminated to ensure that confidentiality of all patients’ records was maintained. The STOP study protocol was approved by the Human Research Committee of the Women’s and Children’s Hospital Adelaide Australia (HREC/14/WCHN/90).19