Monovalent COVID-19 booster vaccinations administered in early pregnancy (before 20 weeks gestation) did not raise the risk of miscarriage, two studies have shown.

The studies add to the growing understanding of the safety of COVID-19 booster vaccinations among people who are pregnant.

The researchers from HealthPartners Institute in Minneapolis, US, analysed data from more than 100,000 pregnancies between six and 19 weeks gestation.

The data was collected between November 1, 2021, and June 12, 2022. Researchers found, using a 28 or 42-day window, that receipt of a booster vaccination was not associated with miscarriage.

"COVID infection during pregnancy increases the risk of poor outcomes, yet many people who are pregnant or thinking about getting pregnant are hesitant to get a booster dose because of questions about safety," said Elyse Kharbanda, senior investigator at HealthPartners Institute and lead author of the study.

"Our data supports the safety of booster vaccination in early pregnancy," Kharbanda added, in the study published in JAMA Network Open.

Another study, published in the journal Obstetrics and Gynecology, also led by HealthPartners Institute, showed that COVID booster vaccination at any point during pregnancy was not associated with increased risk for serious acute adverse events.

In that study, researchers evaluated data from more than 80,000 pregnancies that occurred between September 23, 2021, and June 30, 2022.

Booster vaccination in pregnancy did not increase risks for thrombocytopenia, myocarditis, venous thromboembolism, ischemic stroke, or other serious adverse events within 21 or 42 days after vaccination.

"We continue to find that COVID-19 vaccinations in pregnancy are safe," said Malini DeSilva, investigator at HealthPartners Institute and lead author of the study.

"Ongoing vaccine surveillance work is important because it provides reassurance and helps people feel confident in their vaccinations," DeSilva said.