No Link Between COVID-19 Vaccination During Pregnancy and Higher Risk of Preterm Birth or Stillbirth

Vaccination against COVID-19 during pregnancy is not associated with a higher risk of preterm birth, small for gestational age at birth, or stillbirth, concludes a new population based retrospective cohort study led by the CHEO Research Institute and the University of Ottawa's Faculty of Medicine.

These findings, published recently in The BMJnorth_eastexternal link, can help inform evidence based decision making about the risks and benefits of COVID-19 vaccination during pregnancy.

“Our study found no evidence of increased risk of preterm birth, very preterm birth, small-for gestational-age at birth, or still birth following COVID-19 vaccination during pregnancy. The results of this study provide further evidence for care providers and pregnant people about the safety of COVID-19 vaccination during pregnancy," said lead author Dr. Deshayne Fell, Scientist at the CHEO Research Institute and Associate Professor in the University of Ottawa's Faculty of Medicine.

SARS-CoV-2 infection during pregnancy has been associated with higher risks of complications, including admission to hospital and death for pregnant individuals, as well as preterm birth and stillbirth. COVID-19 vaccination during pregnancy has been shown to be effective against COVID-19 in pregnant individuals as well as their newborns, but evidence about pregnancy outcomes after COVID-19 vaccination during pregnancy from large studies is limited.

In a project supported by the Public Health Agency of Canada through the Vaccine Surveillance Reference Group and the COVID-19 Immunity Task Force, researchers set out to assess the risk of preterm birth, small for gestational age at birth, and stillbirth after COVID-19 vaccination during pregnancy.

They used the BORN Ontario birth registry, the provincial birth registry based out of CHEO, to identify all liveborn and stillborn infants with a gestational age of at least 20 weeks or birth weight of at least 500g in Ontario, Canada between May 1 and Dec. 31, 2021. This information was then linked to the COVaxON, the province’s COVID-19 immunization database.

A wide range of potentially influential factors were taken into account, including the mother’s age at delivery, pre-pregnancy body mass index, reported smoking or substance use during pregnancy, pre-existing health conditions, number of previous live births and stillbirths, area of residence and income.

Of 85,162 births, 43,099 occurred in individuals who received one dose or more of a COVID-19 vaccine during pregnancy - 42, 979 (99.7%) received an mRNA vaccine, mainly Pfizer-BioNTech or Moderna. (See Figure 1)

The study found that vaccination during pregnancy was not associated with any increased risk of overall preterm birth (6.5% among vaccinated v 6.9% among unvaccinated), spontaneous preterm birth (3.7% v 4.4%), or very preterm birth (0.59% v 0.89%). No increase was found in risk of small for gestational age at birth (9.1% v 9.2%) or stillbirth (0.25% v 0.44%).

Findings were similar irrespective of what stage (trimester) of pregnancy vaccination was given, number of doses received during pregnancy, or which mRNA vaccine product was received. There are still some important unanswered questions in need of future research, such as assessing COVID-19 vaccination before pregnancy or around the time of conception and assessment of non-mRNA vaccines used during pregnancy.

Researchers noted “Future studies to assess similar outcomes after immunisation with nonmRNA COVID-19 vaccine types during pregnancy should be a research priority.”

Reference: Fell DB, Dimanlig-Cruz S, Regan AK, et al. Risk of preterm birth, small for gestational age at birth, and stillbirth after covid-19 vaccination during pregnancy: population based retrospective cohort study. BMJ. 2022;378:e071416. doi: 10.1136/bmj-2022-071416

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