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COVID-19 vaccine side-effects less likely in pregnant people, says study

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A basket of needles containing Pfizer-BioNTech COVID-19 vaccine waits to be administered to patients at a COVID-19 clinic in Ottawa on March 30, 2021. A new Canadian study suggests that pregnant people experienced lower rates of health problems after receiving the COVID-19 vaccine than their counterparts who weren't pregnant. THE CANADIAN PRESS/Sean Kilpatrick

Pregnant people experienced lower rates of side-effects from the COVID-19 vaccine than their counterparts who weren't pregnant, a new Canadian study suggests.

The Canadian National Vaccine Safety Network collected data from 191,360 vaccinated women aged 15 to 49 between December 2020 and November 2021. The researchers asked participants to report "significant health events" that were serious enough to make them miss school or work, seek medical attention or disrupt their routines.

Of 5,597 pregnant participants, four per cent reported a significant health event within seven days of receiving their first dose of an mRNA vaccine, and 7.3 per cent of 3,108 pregnant respondents said they had side-effects from their second shots.

Among those who weren't pregnant, 6.3 per cent of 174,765 respondents reported a significant health event after dose one, and 11.3 per cent of 10,254 participants said they felt sick after dose two.

"One of the things that was really striking was that the rates of these events happening in pregnant people was lower than the rates happening in non-pregnant people at the same age," said Manish Sadarangani, lead author of the paper published in the Lancet Infectious Diseases journal on Thursday. "It's very reassuring around the safety of COVID vaccines and pregnancy."

Studies on other vaccines have found that pregnant people experience side-effects at roughly the same rate as those who aren't pregnant or even slightly higher, said Sadarangani, an investigator at BC Children's Hospital.

More research is needed to understand why this might not be the case for mRNA COVID-19 vaccines, Sadarangani said, but he suspects the physical transformation of pregnancy could be a factor.

"There's a lot of hormonal and immunological and physiological changes happening during pregnancy, and some of them we understand, some of them we don't," he said. "I'm presuming that some of these changes are leading to these lower rates."

Thursday's study found that rates of serious health events after getting a COVID-19 vaccine, such as hospitalization, were similarly rare across all groups. 

There was no significant difference in the rates of miscarriage or stillbirth among participants who were vaccinated and those who weren't.

Researchers are conducting a followup survey to see if participants experienced any side-effects six months after their COVID-19 shots, Sadarangani said.

Pregnant people are at increased risk of COVID-19 complications, he said, so it's all the more important that researchers continue to study how vaccination affects them and their babies.

"All of the data we have really highlight the safety of all of these vaccines in pregnancy," said Sadarangani. "Ultimately, this is the best way to protect this group of people in our population."

This report by The Canadian Press was first published Aug. 12, 2022.

Adina Bresge, The Canadian Press