Covid-19 mRNA vaccines are safe to use in pregnancy, and result in lower rates of health events in pregnant women, compared to similarly aged non-pregnant vaccinated people, according to a large Canadian study published in The Lancet Infectious Diseases journal.

Pregnant women are at higher risk of severe Covid-19 disease compared to similarly aged non-pregnant individuals. Based on established prior safety of inactivated vaccines in pregnancy and reassuring data from the small number of pregnancies occurring during pre-authorisation vaccine trials, Covid-19 vaccines were recommended for use in pregnancy in many countries early on in vaccine deployment. 

What Makes The Study Special?

The Canadian study is one of the first to look at vaccine side effects in a group of vaccinated pregnant women and at the same time, observe an unvaccinated pregnant group and a vaccinated non-pregnant group to enable comparisons between the three groups, according to the authors. 

The study was conducted by The Canadian National Vaccine Safety (CANVAS) Network. 

Dr Manish Sadarangani from the British Columbia Children’s Hospital Research Institute, and first author on the paper, said that in early stages of the Covid-19 vaccine rollout, there was low vaccine uptake among pregnant people due to concerns about data availability and vaccine safety. He added that there still is lower than average uptake among non-pregnant women of reproductive age.

Sadarangani further said that large, observational studies like the new Canadian study are crucial for proper understanding of the rates of adverse health events in pregnant women after different doses of Covid-19 vaccination, and this information should be used to inform pregnant women about the side effects they may experience in the week following vaccination.

Data From Participants Across Seven Canadian Provinces Analysed

Between December 2020 and November 2021, the study authors analysed data from participants across seven Canadian provinces and territories. As part of the study, the participants were asked to report every health event they experienced during the seven days following each dose of Covid-19 vaccination. 

The unvaccinated pregnant women served as the control group, and were asked to record any health problems over the seven days before they filled out the survey. 

A total of 1,91,360 women aged 15 to 49 years with known pregnancy status completed the first dose survey, and 94,937 women completed the second dose survey.

Significant And Serious Health Events

According to the study, a ‘significant health event’ was defined as a new or worse health event which was enough to cause the woman to miss school or work, require medical consultation, or prevent daily activities in the previous seven days. 

Meanwhile, a ‘serious health event’ was defined as any event resulting in an emergency department visit or hospitalisation in the previous seven days.

Of 5,597 mRNA-vaccinated pregnant females, 226 reported a significant health event within seven days after the first dose of an mRNA vaccine, accounting for four per cent of vaccinated pregnant women. 

Meanwhile, of 3,108 pregnant women who had received both doses, 227 reported a significant health event within seven days after the second dose.  

In pregnant females, the most significant health events after the second dose were a general feeling of being unwell, respiratory tract infection, and a headache or migraine.

Of 339 pregnant unvaccinated women, 11 reported similar significant health events in the seven days prior to survey completion, accounting for 3.2 per cent of pregnant unvaccinated women. Meanwhile, of 1,74,765 females in the vaccinated non-pregnant control group, 10,950 participants reported a significant health event in the week after the first dose, accounting for 6.3 per cent of women in the control group. About 11.3 per cent of the women in the control group reported a significant health event after the second dose.

Serious Health Events Rare

In all the groups, serious health events were rare. Less than one per cent of women reported serious health events. 

According to the study, serious health events occurred at similar rates in vaccinated pregnant females, vaccinated non-pregnant women, and unvaccinated controls after the first dose and the second dose.

The most frequently reported adverse pregnancy outcome, with no significant difference between the rates in vaccinated and unvaccinated women, was miscarriage or stillbirth. 

Of 339 unvaccinated pregnant women, seven experienced a miscarriage or stillbirth within seven days after the first dose of any mRNA vaccine, accounting for 2.1 per cent of the unvaccinated pregnant females. 

Meanwhile, of 5,597 vaccinated pregnant women, 83 experienced a miscarriage or stillbirth within seven days after the first dose of any mRNA vaccine, accounting for 1.5 per cent of vaccinated pregnant women.

Dr Julie Bettinger from the British Columbia Children’s Hospital Research Institute, and senior author on the paper, said the lower rate of significant health events amongst vaccinated pregnant individuals, compared with vaccinated non-pregnant individuals, is unexpected and requires more research. She stated that previous studies on other vaccines in pregnant women have mostly reported no significant differences in health events between pregnant and non-pregnant women or have found higher rates in pregnancy. 

Bettinger added that further studies of non-Covid-19 mRNA vaccines are required to identify if the reduced side effects observed in pregnant people in the study is a feature of the mRNA vaccine platform, or of Covid-19 mRNA vaccines. 


The authors noted some limitations to the study, including the fact that most participants who reported ethnicity in the study were white and therefore, the data may not be entirely generalised to other populations. Also, the study cannot conclude about longer term reactions of vaccines because it focused on health events occurring within the first seven days following vaccination. The data are not completely reliable because they are based on self-reports from study participants, without verification by medical records. Researchers are conducting a longer-term follow-up of the cohort.

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