Major Canadian study confirms mRNA COVID vaccines safe in pregnancy

A large Canadian study tracking more than 180,000 women has found no evidence that mRNA COVID-19 vaccines increase the risk of miscarriage, stillbirth, or other serious pregnancy complications. The research, published this week in the Lancet Infectious Diseases, compared outcomes in vaccinated pregnant women, unvaccinated pregnant women, and vaccinated women who were not pregnant—making it one of the most comprehensive examinations of vaccine safety during pregnancy conducted to date.

The timing of the study matters because pregnant women have consistently shown lower vaccination rates than the general population, even as evidence has mounted that COVID-19 infection itself poses substantial risks to both mother and fetus. Julie Bettinger, a pediatrics professor at the University of British Columbia and the study's lead author, emphasized the stakes plainly: infection during pregnancy carries serious consequences for maternal health and can harm fetal development. The vaccine, by contrast, appears to carry no such burden.

The researchers drew on data from seven provinces and tracked two vaccination doses across their participant groups. Nearly 6,000 pregnant women received at least one dose of an mRNA vaccine, with more than 3,100 completing the two-dose series. The unvaccinated control group included 339 pregnant women and 5,840 who were not pregnant. Participants recorded any significant health events—those affecting work, school, or daily life—within a week of vaccination, as well as any serious events requiring emergency care or hospitalization.

The findings on side effects showed a modest difference between groups. Four percent of vaccinated pregnant women reported a significant health event after their first dose; that rose to 7.3 percent after the second. Among unvaccinated pregnant women, 3.2 percent reported significant events in the same timeframe. The most common complaints were general malaise, muscle aches, headaches, and respiratory infections—the typical post-vaccination experience. Serious health events requiring emergency intervention were rare in both groups, with no meaningful difference between them.

On the central question—whether vaccination harmed pregnancy outcomes—the data showed reassurance. Miscarriage or stillbirth was the most common adverse pregnancy outcome tracked. In the unvaccinated group, 2.1 percent reported such a loss within the previous week; in the vaccinated group, 1.5 percent did. Nearly 90 percent of all recorded pregnancy losses occurred in the first trimester, the period when most miscarriages naturally occur. The vaccinated group's rate was actually lower, though researchers were careful not to overstate causation from this observational data.

One unexpected finding emerged when researchers compared vaccinated pregnant women to vaccinated non-pregnant women. The non-pregnant group reported more side effects serious enough to interfere with daily function—6.3 percent after the first dose and 11.3 percent after the second, compared with 4 and 7.3 percent in the pregnant group. Why pregnancy appeared to buffer against certain vaccine side effects remains unclear and will require further investigation, Bettinger said.

The study was funded by the Canadian Institutes of Health Research and the Public Health Agency of Canada. Some participants dropped out during follow-up—a common challenge in longitudinal research that Bettinger attributed to various factors, including survey non-response, voluntary withdrawal, and the demands of new parenthood. The research team is continuing long-term follow-up and plans to publish additional findings as the data accumulates, potentially offering even longer windows into vaccine safety during and after pregnancy.