Safety of COVID-19 mRNA Vaccines in Pregnancy Confirmed in Large Study

The rate of health events was lower among pregnant persons than among nonpregnant female individuals following COVID-19 mRNA vaccinations.

The safety of COVID-19 mRNA vaccine use in pregnancy was confirmed in a large Canadian study published in The Lancet Infectious Diseases. The rate of postvaccination health events with mRNA vaccine use was lower among pregnant persons than among similarly aged, nonpregnant female individuals, the findings showed.

“Large, observational studies like ours are crucial for proper understanding of the rates of adverse health events in pregnant persons after different doses of COVID-19 vaccination,” said lead study author Manish Sadarangani, MRCPCH, DPhil, BMBCh, MA, an investigator at the Vaccine Evaluation Center (VEC) at British Columbia Children’s Hospital Research Institute. “This information should be used to inform pregnant persons about the side effects they may experience in the week following vaccination.”

Pregnant persons are at higher risk of severe COVID-19 disease compared with similarly aged nonpregnant individuals. However, uptake of the vaccine in the early stages of the COVID-19 vaccine rollout was low due to concerns about vaccine safety, Dr Sadarangani explained. This study from The Canadian National Vaccine Safety (CANVAS) Network is one of the first to compare vaccine side effects among vaccinated pregnant persons with those among unvaccinated pregnant persons and vaccinated nonpregnant female individuals.

Study Design

The CANVAS researchers examined data from participants across 7 Canadian provinces and territories between December 2020 and November 2021. All vaccinated participants were asked to self-report any health events during the 7 days following each dose of BNT162b2 (Pfizer) or mRNA-1273 (Moderna) vaccines. The unvaccinated pregnant control group was asked to record any health problems over the 7 days before they filled out the survey. In total, 191,360 women aged 15 to 49 years with known pregnancy status completed the survey after dose 1 and 94,937 completed the survey after dose 2.

[Findings] are consistent with and add to the growing body of evidence that COVID-19 mRNA vaccines are safe during pregnancy.

A significant health event was defined as a new or worse health event that caused the participant to miss school/work, require medical consultation, and/or prevent daily activities in the previous 7 days. A serious health event was defined as any event resulting in an emergency department visit and/or hospitalization in the previous 7 days.

Rates of Significant Health Events With mRNA COVID-19 Vaccines

Significant health events following 1 dose of an mRNA COVID-19 vaccine were reported by 4.0% of pregnant persons after dose 1 and 7.3% after dose 2 compared with 6.3% and 11.3%, respectively among vaccinated nonpregnant female participants. The rate was 3.2% (11/339) among pregnant unvaccinated participants in the past 7 days.

Further analysis by mRNA vaccine type (mRNA-1273 or BNT162b2) in the pregnant vaccinated group showed similar rates of health events after dose 1 of either vaccine (4.0% and 4.1%, respectively) but higher rates for dose 2 of the mRNA-1273 vaccine compared with dose 2 of BNT162b2 (12.% vs 4.2%). Pregnant vaccinated persons had an increased odds of a significant health event after dose 2 of mRNA-1273 (adjusted odds ratio [aOR], 4.4 [95% CI, 2.4-8.3]) compared with pregnant unvaccinated controls.

Table. Rate of Health Eventsa Following COVID-19 mRNA Vaccine Doses

GroupAny mRNA Dose 1Any mRNA Dose 2
Pregnant, vaccinated4.0% (226/5597)7.3% (227/3108)
Nonpregnant, vaccinated6.3% (10,950/174,765)11.3% (10,254/91,131)
aHealth events were defined as new or worsening of a health event sufficient to cause work or school absenteeism, medical consultation, or prevent daily activities.

The most common of these health events after dose 2 in pregnant vaccinated women were a general feeling of being unwell, headache/migraine, and respiratory tract infection. Serious health events were rare in all groups (<1%) and occurred at similar rates in all 3 groups.

Miscarriage/stillbirth was the most frequently reported adverse pregnancy outcome with no significant difference between the rates in vaccinated and unvaccinated women; 2.1% of unvaccinated pregnant women and 1.5% of vaccinated pregnant women experienced a miscarriage or stillbirth within 7 days after dose 1 of any mRNA vaccine.

“The lower rate of significant health events amongst vaccinated pregnant people, compared with vaccinated nonpregnant individuals, is unexpected and requires more research,” said senior author Julie Bettinger, MPH, PhD, a vaccine safety scientist at the VEC. “Previous studies on other vaccines in pregnant persons have mostly reported no significant differences in health events between pregnant and nonpregnant females or have found higher rates in pregnancy. Further studies of non-COVID-19 mRNA vaccines are required to identify if the reduced side effects observed in pregnant people in this study is a feature of the mRNA vaccine platform, or of these specific vaccines.”

Study authors noted that a majority of participants who reported their ethnicity were White women; thus, the data may not be generalizable to other populations. Also, data were based on self-reports, without verification by medical records. Additionally, data on health events occurring after the first 7 days following vaccination cannot be determined; however, longer-term follow-up of this cohort is ongoing.

Researchers concluded that the data provide reassuring evidence that COVID-19 mRNA vaccines are safe in pregnancy and call for pregnant people to be provided with accurate information about the possible adverse events after vaccination.

Growing Body of Evidence on Vaccine Safety During Pregnancy

In an accompanying editorial, Sascha Ellington, PhD, and Christine Olson, MPH, from the Centers for Disease Control and Prevention commented that the findings “are consistent with and add to the growing body of evidence that COVID-19 mRNA vaccines are safe during pregnancy.” They added that COVID-19 vaccination rates among pregnant people continue to be lower than those of nonpregnant female individuals of reproductive age. “Given the risks of significant illness and adverse pregnancy outcomes, it is imperative that we continue to collect and disseminate data on the safety and effectiveness of COVID-19 vaccination in pregnancy and to encourage healthcare providers to promote vaccination during all trimesters of pregnancy.”

This article originally appeared on Clinical Advisor


  1. Sadarangani M, Soe P, Shulha HP, et al; Canadian Immunization Research Network. Safety of COVID-19 vaccines in pregnancy: a Canadian National Vaccine Safety (CANVAS) network cohort study. Lancet Infect Dis. 2022 Aug 11:S1473-3099(22)00426-1. doi: 10.1016/S1473-3099(22)00426-1
  2. COVID-19 mRNA vaccines are safe in pregnancy, large study confirms. News release. Lancet. August 12, 2022. Accessed August 16, 2022.