Study says COVID mRNA vaccines safe during pregnancy — 10 key points

"COVID-19 mRNA vaccines have a good safety profile in pregnancy," it said in conclusion.

In this study, researchers also focused on the side effects in a group of vaccinated pregnant women simultaneously as both an unvaccinated pregnant group and a vaccinated non-pregnant group. Rates of significant adverse events following immunisation were highest after the second dose for Moderna recipients. However, it said, "both mRNA vaccines are highly immunogenic and effective in pregnancy."

Overall, 226 (4·0 percent) of 5,597 mRNA-vaccinated pregnant females reported a significant health event within seven days after dose one of an mRNA vaccine, and 227 (7·3 percent) of 3,108 after dose two.

The health event rates were similar after dose one for both mRNA vaccines — 137 of 3,414 for Pfizer BioNTech and 89 of 2183 for Moderna — but higher for Moderna than Pfizer after dose two.

The most common significant health events after the second dose of Moderna in pregnant women are feeling unwell or malaise or myalgia (139 of 1,216), headache or migraine (103 of 1,216), and respiratory tract infection (68 of 1,216).

Among pregnant and vaccinated participants who reported significant health events, most of them recognised their symptoms within 24 hours following vaccination — 57·5 percent of 226 after dose one and 82·4 percent of 227 after dose two of an mRNA vaccine. The majority — 54·9 percent of 226 after dose one and 74·0 percent of 227 —  resolved within three days.

In comparison, 3·2 percent of 339 pregnant unvaccinated (control group) participants reported similar events in the seven days before survey completion. Of these, 18·2 percent reported their symptoms as resolved at least 24 hours before the control survey and 81·8 percent reported their events as ongoing for at least six days.

The reported serious health events were rare — reported among eight (0·4 percent) of 1,892 and 11 (0·9 percent) of 1216 across various pregnant groups. This occurred at similar rates in vaccinated pregnant individuals and unvaccinated controls and after doses one and two for all vaccine types.

Miscarriage or stillbirth was the most frequently reported adverse pregnancy outcome, and it was reported at similar rates between control (2·1 percent of 339) and vaccinated groups within seven days after dose one of any mRNA vaccine. Almost all pregnancy losses occurred during the first trimester.

Other adverse pregnancy outcomes such as vaginal bleeding, abnormal fetal heart rate, and reduced fetal movement were rarely reported within seven days following any mRNA vaccination.

The first dose of any mRNA vaccine and either dose of Pfizer were not associated with an increased risk of significant health events. Similarly, the researchers found "no significant association between vaccination status and serious health events in pregnant people".

The increase in significant health events after dose two of any mRNA vaccine was no longer observed when researchers restricted their outcome variable to events requiring medical consultation. "When restricted to events resulting in medical consultation, there was no difference between groups in any of the analyses."

Pregnant people in this study reported high rates of injection site pain (92 percent after dose two), fatigue (72 percent), headache (55 percent), myalgia (54 percent) and fever or chills (35–37 percent), with higher rates of adverse events after dose two than after dose one.

"...We found that significant health events — new or worsening health events following vaccination sufficient to cause work or school absenteeism, medical consultation, or prevent daily activities — were lower in pregnant people than in age-matched non-pregnant vaccine recipients," the study said.

Researchers said the data could be used to appropriately inform pregnant people regarding the "reactogenicity of COVID-19 vaccines" during pregnancy. The finding should also be considered alongside effectiveness and immunogenicity data to make appropriate recommendations about the "best use of COVID-19 vaccines in pregnancy".

The Pfizer-BioNTech and Moderna COVID-19 vaccines are messenger RNA (mRNA) vaccines. According to the CDC, many vaccines put a weakened or inactivated germ into our bodies to trigger an immune response.

But this is not the case with mRNA vaccines. These vaccines use "mRNA created in a laboratory to teach our cells how to make a protein — or even just a piece of a protein — that triggers an immune response inside our bodies".

This immune response, which produces antibodies, is what helps protect people from getting sick from that germ in the future.