Coronavirus particles, illustration - SARS-CoV-2 virus

A large Canadian prospective study involving nearly 200,000 women has provided reassuring evidence as to the safety of COVID-19 messenger mRNA vaccination in pregnancy.

Indeed, pregnant women in the study experienced lower rates of health events overall after vaccination than similarly aged, non-pregnant vaccinated women, researchers report in The Lancet Infectious Diseases.

Further analysis among vaccinated women indicated an increased risk of significant health events among those who were pregnant within a week of the second dose of the primary series mRNA-1273 (Moderna) vaccine that was not observed with either dose of BNT162b2 (Pfizer BioNTech).

These events included either needing to miss school or work, requiring medical consultation or not being able to take part in daily activities within the previous week.

However, when the analysis was restricted to more serious events requiring an emergency department visit or hospitalisation in the previous week, the increased risk was not observed.

Researcher Manish Sadarangani, an associate professor at the University of British Columbia in Vancouver, Canada, and co-workers note that both mRNA vaccines are highly immunogenic and effective in pregnancy.

They add: “Given the increased rate of significant complications associated with COVID-19 in pregnancy, high vaccine coverage in this group is important for protection of the pregnant individual and young infant, via passive transplacental transfer of antigen-specific IgG antibody and protection via breast milk.”

The Canadian National Vaccine Safety (CANVAS) Network study compared 5625 pregnant, vaccinated women aged 15–49 years with 185,735 nonpregnant vaccinated women and 339 pregnant unvaccinated control women of a similar age.

Overall, 4% of 5597 vaccinated pregnant people reported a significant health event after dose 1 of mRNA-1273 or BNT162b2 vaccination, with similar adverse events for both vaccines.

In addition, 7·3% of 3108 vaccinated pregnant women reported a significant health event after a second dose of a primary series vaccine. This did differ by vaccine type, with 12·1% of 1216  pregnant women reporting a significant health event after dose two of mRNA-1273 versus 4·2% of 1892 after the second dose of BNT162b2.

The most common significant health events following both doses in pregnant people were malaise or myalgia (3·5% for two doses of BNT162b2 vs 11·4% for two doses of mRNA-1273).

Serious health events were rare and occurred in less than one percent of any group.

The most frequently reported adverse pregnancy outcome was the combined outcome of miscarriage and stillbirth, which did not significantly differ in frequency between vaccinated and unvaccinated women (1.4% vs 2.1%).

Multivariate analysis revealed that pregnancy was associated overall with reduced odds of significant adverse events reported in the week following receipt of each dose, with an adjusted odds ratio of 0.63 for any first mRNA dose and 0.62 for any second mRNA dose.

Additionally, pregnancy was not associated with increased risk of serious adverse events following immunisation after either dose of BNT162b2 or the first dose of mRNA-1273.

However, there but was an increased risk following the second dose of mRNA-1273, with an adjusted odds ratio of 2.3.  Among 1216 pregnant women  who received the second dose of mRNA-1273, 0·9% reported a serious adverse event within a week.

However, the increase in significant health events after dose two of any mRNA vaccine and mRNA-1273 was no longer observed when we restricted our outcome variable to events requiring medical consultation.

In an accompanying Comment article, Sascha Ellington and Christine Olson from the Centers for Disease Control and Prevention in Atlanta, Georgia, USA, write: “These findings are consistent with and add to the growing body of evidence that COVID-19 mRNA vaccines are safe during pregnancy.”

They add: “COVID-19 vaccination among pregnant people continues to be lower than among non-pregnant females of reproductive age. Given the risks of severe illness and adverse pregnancy outcomes, continuing to collect and disseminate data on the safety and effectiveness of COVID-19 vaccination in pregnancy and encouraging health-care providers to promote vaccination during all trimesters of pregnancy is imperative.”

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