Safety of third SARS-CoV-2 vaccine (booster dose) during pregnancy

COVID-19 during pregnancy is associated with adverse outcomes for both the mother and fetus. SARS-CoV-2 vaccination has significantly reduced the risk for symptomatic disease. Several studies have reported on the safety of SARS-CoV-2 vaccination during pregnancy, with no adverse effects on the obstetrical outcomes. However, data regarding the obstetrical outcomes following a booster dose of the SARS CoV-2 vaccination during pregnancy have not yet to be published.

This study aimed to examine the association between the booster dose of the SARS CoV-2 vaccination during pregnancy and obstetrical outcomes.

This was a retrospective cohort study of women who delivered between July and October 2021 at a large tertiary medical center. We compared women who received the booster vaccination dose during pregnancy with women who were not vaccinated and with those who only received 2 vaccination doses. Primary outcomes were the incidence of preterm labor and of small for gestational age neonates. Secondary outcomes were other maternal and neonatal complications. A secondary analysis investigating the association between the time from vaccination to delivery and the outcomes was also performed. Multivariable logistic regression models were used to adjust for potential confounders.

There were 6507 women who met the inclusion criteria: 294 women received 3 doses of the vaccination, 2845 women received only 2 doses, and 3368 were unvaccinated. Patients receiving 3 doses of the vaccine were older and more likely to smoke than unvaccinated patients. No differences were noted among the triple-vaccinated, twice-vaccinated, and unvaccinated groups with regards to preterm birth and the incidence of small for gestational age neonates. Regarding the secondary outcomes, women in the triple-vaccinated group had higher rates of postpartum hemorrhage (9.5% vs 3.21%; P<.001) and gestational diabetes mellitus (12.2% vs 8.3%; P=.02) and were less likely to have hypertensive disorders of pregnancy (0% vs 1.4%; P=.041) than the unvaccinated group. Compared with the twice-vaccinated patients, patients with 3 doses of the vaccine were more likely to experience postpartum hemorrhage (9.5% vs 3.5%; P<.001) and were less likely to have a low umbilical artery pH (0.7% vs 6.1%; P<.001). In the sensitivity analysis comparing patients who delivered within 2 weeks of the third vaccination dose (n=53) with those who delivered at least 6 weeks after vaccination (n=96), there were no differences in the rates of small for gestational age neonates, preterm birth, postpartum hemorrhage, or cesarean delivery.

Receiving the booster dose of the SARS-CoV-2 vaccination during pregnancy was not associated with adverse obstetrical outcomes when compared with unvaccinated or twice-vaccinated women. However, higher rates of postpartum hemorrhage were observed. Further studies on a larger scale are needed to confirm these findings.