Informing Maternal Immunization Trials: An Observational Study to Assess Background Rates of Maternal, Pregnancy, and...

Pregnant individuals have been historically excluded from clinical trials. To evaluate the safety of vaccines in pregnant individuals and their infants, more data on expected ranges of background rates of pregnancy-related outcomes of interest are needed. The objective of this real-world observational study was to assess background rates of maternal, pregnancy and neonatal/infant outcomes in pregnant individuals and their infants.

We conducted a retrospective observational cohort study of pregnant individuals and their infants using data from Kaiser Permanente Northern California from January 1, 2007, to December 31, 2018. We assessed the incidence proportions of 17 maternal, 4 pregnancy and 10 neonatal/infant outcomes of interest, including eclampsia, preterm labor and stillbirths. Incidence proportions were stratified by maternal age group, race/ethnicity and insurance type.

There were 415,074 singleton pregnancies and 413,488 singleton live births included in the study. The most common adverse outcomes (>10%) were preterm labor, gestational diabetes, minor congenital anomalies, macrosomia and large for gestational age. The proportions for pregnancy-related hypertension, fetal asphyxia and preeclampsia were also relatively high (between 5% and 7%). Pregnant individuals (and their infants) in the youngest and oldest age groups, and those of Black race, had the highest proportions for nearly all outcomes.

Overall, the proportions of the outcomes assessed were generally consistent with previously published literature of pregnant individuals and their infants in the United States (US). The study provides background rates of important maternal, pregnancy and neonatal/infant outcomes in a US population that may inform vaccine clinical trials and vaccine safety studies in pregnant individuals.