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A pregnant woman receives a vaccine for COVID-19 in Schwenksville, Pa. on Feb. 11, 2021.HANNAH BEIER/Reuters

Pregnant women who received mRNA COVID-19 vaccines were not more likely to experience miscarriage, stillbirth or other severe health events compared with unvaccinated pregnant women, according to a major new Canadian study.

The findings are important, as COVID-19 infection during pregnancy is linked to a markedly higher incidence of serious health complications, but evidence shows pregnant women have a lower vaccination rate than the general population.

“If you get sick with the infection, it actually has really adverse effects on pregnant people,” said Julie Bettinger, senior study author and a pediatrics professor at the University of British Columbia, adding that it can also harm fetal development. “It’s not good for the mom, it’s not good for the baby.” (The study used the terms “pregnant people” and “pregnant women” interchangeably.)

The study also found that, compared with vaccinated non-pregnant people, women who were vaccinated during pregnancy were less likely to experience side effects serious enough to interfere with school or work, such as fatigue or migraine, but not severe enough to require an emergency room visit or hospitalization, a finding that surprised researchers.

The study, published Wednesday in the journal Lancet Infectious Diseases, is one of the largest to look at COVID-19 vaccines in pregnancy and the first to compare the rate of possible side effects in unvaccinated and vaccinated pregnant women, as well as a control group of vaccinated people who were not pregnant. Dr. Bettinger worked with colleagues who, together, form the Canadian National Vaccine Safety Network.

The researchers used data from thousands of women in seven provinces. The findings are based on more than 180,000 women aged 15 to 49 who received one dose of an mRNA vaccine and more than 94,000 who received a second dose. Of the pregnant participants, nearly 6,000 had received one dose, and more than 3,100 had received two doses. A control group of unvaccinated women – 339 pregnant and 5,840 not pregnant – were also included in the study.

Some of the study participants were lost during follow-up, which is why the number of second-dose participants is lower. Dr. Bettinger said there are likely a variety of reasons why they didn’t complete the second portion, such as not receiving the study survey or deciding to drop out. It’s also likely some of the participants gave birth, which left them too busy to participate, she added.

Women were asked to record any significant or serious health events that occurred within one week of being vaccinated. The unvaccinated control group was asked to record any health events that had occurred in the previous seven days. Significant health events were defined as those that could interfere with school, work or daily activities or require consultation with a medical professional. Serious health events, meaning those that required an emergency department visit or hospitalization, were also recorded.

The findings show that 4 per cent of vaccinated pregnant women reported a significant health event within seven days of their first dose; 7.3 reported a significant health event after their second. The most common symptoms reported included feeling unwell, muscle aches, headache and respiratory tract infection.

In the unvaccinated pregnant group, 3.2 per cent reported significant health events in the previous seven days.

Serious health events were rare in both the vaccinated and unvaccinated pregnant groups, and there were no major differences. Miscarriage or stillbirth was the most common adverse pregnancy outcome reported. In the unvaccinated group, 2.1 per cent reported a miscarriage or stillbirth in the previous seven days, compared with 1.5 per cent in the vaccinated group. Nearly 90 per cent of all recorded pregnancy losses occurred during the first trimester.

Vaccinated non-pregnant study participants were more likely to report serious health events compared with vaccinated pregnant participants. Overall, 6.3 per cent of non-pregnant women reported a serious health event within seven days of their first dose and 11.3 per cent reported one within a week of their second.

It’s not entirely clear why this group was more likely to report some side effects than the pregnant and vaccinated group, Dr. Bettinger said, and will require further study.

The researchers are continuing with long-term follow-up on the study participants and plan to report those findings as well.

The study received funding from the Canadian Institutes of Health Research and the Public Health Agency of Canada.

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