Health events among pregnant women after COVID-19 vaccination
Health events among pregnant women after COVID-19 vaccination

Health events among pregnant women after COVID-19 vaccination

In a recent study posted to medRxiv* preprint server, researchers demonstrate that messenger ribonucleic acid (mRNA) -based vaccines against coronavirus disease 2019 (COVID-19) are safe during pregnancy, with lower incidences of significant side effects after immunization (AEFIs) in pregnant women than not -pregnant women.

Examination: Safety of COVID-19 Vaccines During Pregnancy: A Canadian National Vaccine Safety (CANVAS) Network Study. Image credit: Huseyin Eren Obuz / Shutterstock


Several research institutes have published positive recommendations for mRNA-based COVID-19 vaccines during pregnancy, based on evidence of high efficiency in clinical trials prior to approval. However, in the absence of a concurrent control group to allow comparison with background rates of AEFIs and comparisons based solely on historical rates of AEFIs, concerns still lurk about the safety of mRNA vaccines during pregnancy.

The Canadian National Vaccine Safety Network (CANVAS), established during the 2009 influenza pandemic, has been monitoring COVID-19 vaccine safety in Canada since the rollout of the vaccine in December 2020 to provide rapid real-time safety data.

CANVAS actively monitors persons with significant health events and enrolls active control group (s) to enable comparisons with unvaccinated persons within a similar time frame.

About the study

In this study, researchers recruited pregnant and non-pregnant women aged 15-49 years from 4 November 2021 under the ‘vaccinated’ and ‘control’ cohorts in Canada to evaluate the safety profile of mRNA-based COVID-19 vaccines.

The females in the vaccinated cohort had received the first dose of a vaccine within seven days before enrolling in the study. They had an active email address and phone number and could communicate in English or French. They reported the occurrence of AEFIs via an email seven days after each dose of the COVID-19 vaccine and seven months after their first vaccine dose. Control group participants were unvaccinated and reported significant health events that occurred seven days, 28 days, and six months after enrollment in the study.

All participants had to report reactions at the injection site; however, only those who stated that they had a significant health incident should provide additional information.

The researchers analyzed two types of exposures for the study analysis:

  1. pregnancy status among vaccinated people.

Two endpoints were analyzed, including ‘significant’ and ‘serious’ health events, including common and uncommon symptoms following the first and second doses of COVID-19 vaccines. The former is defined as a new or worsening of a health event sufficient to cause work/school absenteeism or medical consultation in the previous seven days, and the latter describes any event resulting in hospitalization.

Likewise, they analyzed three vaccine groups:

  1. BNT162b2,
  2. mRNA-1273, and
  3. any mRNA vaccine.

They also examined associations between the outcomes and the exposures, using two sets of univariate/multivariate (MV) logistic regression models. When fitting MV models, they adjusted known or expected covariates such as age group, prior COVID-19 infection, and trimester of pregnancy, as appropriate.

Lastly, they conducted two sensitivity analyses to evaluate the robustness of the findings.

Study findings

Significant health events were lower in pregnant people than in age-matched non-pregnant vaccine recipients. Among pregnant females, AEFI was higher in those who received the second dose of the mRNA-1273 vaccine. However, there was no difference in AEFIs after either dose of the BNT162b2 vaccine.

Initial clinical trials of the mRNA-1273 and BNT162b2 vaccines have reported relatively high rates of AEFIs compared with most routinely used vaccines, including higher rates for dose two than dose one.

The current study analysis revealed similar patterns among pregnant females. Although the analysis specifically quantified the significant and serious AEFI rates in this population for each of the mRNA vaccines, the lower rate of significant AEFIs among pregnant people, compared with vaccinated non-pregnant females, revealed interesting insights.

During pregnancy, dynamic immunologic adaptations occur, for instance, a skewed response towards a T helper cell 2 (Th2)-dominant state. Since mRNA vaccines have specifically elicited a Th1-biased immune response, the Th2-bias during pregnancy may be partially responsible for this lower rate of significant AEFIs.


Considering the high rate of complications related to COVID-19 in pregnancy, it is crucial to maximize vaccine coverage in this at-high risk population for the protection of both the pregnant female and her young infant. Immunized mothers pass on antigen-specific immunoglobulin G (IgG) antibodies against SARS-CoV-2 via placenta or breast milk.

Overall, the study data appropriately informed about the reactogenicity of COVID-19 vaccines during pregnancy. This information should be considered alongside effectiveness and immunogenicity data to make appropriate recommendations about the best use of COVID-19 vaccines in pregnancy. The long-term data from this cohort following a six-month follow-up, when available, could also prove quite useful.  Similar data from countries where the ChAdOx-S vaccines are used could provide a complete overview of the safety of COVID-19 vaccines in pregnancy.

In the future, research studies should identify whether the observed reduced reactogenicity of non-COVID-19 mRNA vaccines in pregnant people in this study is a feature of the vaccine platform or these specific vaccines.

*Important message

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered as crucial, guide clinical practice / health-related behavior or be treated as established information.

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