The research team wrote in the BMJ that their reassuring findings – drawn from a registry of all births in Ontario over an 8-month period – “can inform evidence-based decision-making” about COVID vaccination during pregnancy.
Previous research has found that pregnant patients are at higher risk of severe complications and death if they become infected with COVID and that vaccination before or during pregnancy prevents such outcomes and reduces the risk of newborn infection, noted Jeffrey Ecker, chief of obstetrics and gynecology at Massachusetts General Hospital, Boston.
This new study “adds to a growing body of information arguing clearly and reassuringly that vaccination during pregnancy is not associated with complications during pregnancy,” said Dr. Ecker, who was not involved in the new study.
He added that it “should help obstetric providers further reassure those who are hesitant that vaccination is safe and best both for the pregnant patient and their pregnancy.”
Methods and results
For the new study, researchers tapped a provincial registry of all live and stillborn infants with a gestational age of at least 20 weeks or birth weight of at least 500 g. Unique health card numbers were used to link birth records to a database of COVID vaccinations.
Of 85,162 infants born from May through December of 2021, 43,099 (50.6%) were born to individuals who received at least one vaccine dose during pregnancy. Among those, 99.7% received an mRNA vaccine such as Pfizer-BioNTech or Moderna.
Vaccination during pregnancy was not associated with greater risk of overall preterm birth (6.5% among vaccinated individuals versus 6.9% among unvaccinated; hazard ratio, 1.02; 95% confidence interval, 0.96-1.08), spontaneous preterm birth (3.7% versus 4.4%; hazard ratio, 0.96; 95% CI, 0.90-1.03) or very preterm birth (0.59% versus 0.89%; hazard ratio, 0.80; 95% CI, 0.67-0.95).
Likewise, no increase was observed in the risk of an infant being small for gestational age at birth (9.1% versus 9.2%; hazard ratio, 0.98; 95% CI, 0.93-1.03).
The researchers observed a reduction in the risk of stillbirth, even after adjusting for potential confounders. Stillbirths occurred in 0.25% of vaccinated individuals, compared with 0.44% of unvaccinated individuals (hazard ratio, 0.65; 95% CI, 0.51-0.84).
A reduced risk of stillbirth – albeit to a smaller degree – was also found in a Scandinavian registry study that included 28,506 babies born to individuals who were vaccinated during pregnancy.
“Collectively, the findings from these two studies are reassuring and are consistent with no increased risk of stillbirth after COVID-19 vaccination during pregnancy. In contrast, COVID-19 disease during pregnancy has been associated with an increased risk of stillbirth,” the researchers wrote.
Findings did not vary by which mRNA vaccine a mother received, the number of doses she received, or the trimester in which a vaccine was given, the researchers reported.
Stillbirth findings will be ‘very reassuring’ for patients
The lead investigator, Deshayne Fell, PhD, said in an interview, the fact that the study comprised the entire population of pregnant people in Ontario during the study period “increases our confidence” about the validity and relevance of the findings for other geographic settings.
Dr. Fell, an associate professor in epidemiology and public health at the University of Ottawa and a scientist at the Children’s Hospital of Eastern Ontario Research Institute, Ottawa, said the evaluation of stillbirth in particular, “a rare but devastating outcome,” will be “very reassuring and useful for clinical counseling.”
A limitation cited by the research team included a lack of data on vaccination prior to pregnancy.
In the new study, Dr, Ecker said, “Though the investigators were able to adjust for many variables they cannot be certain that some unmeasured variable that, accordingly, was not adjusted for does not hide a small risk. This seems very unlikely, however.”
The Canadian research team said similar studies of non-mRNA COVID vaccines “should be a research priority.” However, such studies are not underway in Canada, where only mRNA vaccines are used in pregnancy, Dr. Fell said.
This study was supported by the Public Health Agency of Canada.
Dr. Fell and Dr. Ecker reported no competing financial interests.