Findings add to evidence indicating safety of Covid-19 during pregnancy

Further analysis of data from the CDC’s v-safe Covid-19 vaccine pregnancy registry team showed no increased risk for first- or second-trimester miscarriage associated with the mRNA vaccine.

The cumulative risk of spontaneous abortion from 6 to less than 20 weeks gestation among women who received the Pfizer-BioNTech or Moderna vaccines was around 14% in the primary analysis and 13% in an age-standardized analysis, which was within the range of the background rate reported among the general population of pregnant women.

The findings “suggest that the risk of spontaneous abortion after mRNA Covid-19 vaccination either before conception or during pregnancy is consistent with the expected risk of spontaneous abortion,” wrote Lauren Zauche, PhD, of the CDC’s National Center of Birth Defects and Developmental Disabilities, and colleagues in a letter to the editor.

The new analysis was published online Sept. 8 in The New England Journal of Medicine. The researchers noted that spontaneous abortions occurring at less than 20 weeks of gestation are common, affecting 11% to 22% of recognized pregnancies.

An earlier preliminary analysis of v-safe Covid-19 vaccine pregnancy registry data also showed no obvious safety signals for spontaneous abortion among pregnant women receiving mRNA Covid-19 vaccines, but the data included mostly women vaccinated in their third trimester of pregnancy. In a letter published with the new analysis, the CDC researchers acknowledged that the original analysis was flawed and that the denominator used to calculate risk was not appropriate.

In their updated analysis, Zauche and colleagues used life-table methodology and limited the analysis to 2,456 women who received at least one dose of an mRNA Covid-19 vaccine before 20 weeks of gestation (including prior to conception) and did not have a pregnancy loss before 6 weeks of gestation. The cumulative risk of spontaneous abortion was also age-standardized with existing data on spontaneous abortion by maternal age group.

The researchers conducted a sensitivity analysis to “estimate the maximum possible risk of spontaneous abortion, using an extreme assumption that all participants whose most recent contact was during the first trimester (i.e., at less than 14 weeks of gestation) and whom we were unable to reach during the second trimester experienced a spontaneous abortion immediately after the most recent contact.”

Of the 2,456 registry participants included in the analysis, 2,022 reported ongoing pregnancies at 20 weeks of gestation and 165 reported a spontaneous abortion (154 participants before 14 weeks of gestation). Sixty-five participants with most recent contact during the first trimester could not be reached for second trimester follow-up, 188 completed second trimester follow-up before 20 weeks of gestation, and 16 reported another pregnancy outcome before 20 weeks (including induced abortion or ectopic or molar pregnancy).

The majority of the women included in the analysis were 30 years of age or older (77.3%), were non-Hispanic White (78.3%), and worked in health care (88.8%). Slightly more than half the participants (52.7%) had received the BNT162b2 vaccine (Pfizer–BioNTech).

Among the main findings:

  • The cumulative risk of spontaneous abortion from 6 to less than 20 weeks of gestation was 14.1% (95% CI, 12.1-16.1) in the primary analysis and 12.8% (95% CI, 10.8-14.8) in an analysis using direct maternal age–standardization to the reference population.
  • In the sensitivity analysis, under the extreme assumption that all 65 participants with most recent contact during the first trimester had a spontaneous abortion, the cumulative risk of spontaneous abortion from 6 to less than 20 weeks of gestation was 18.8% (95% CI, 16.6-20.9); after age standardization, the cumulative risk was 18.5% (95% CI, 16.1-20.8).
  • Compared to data from historical cohorts representing the lower and upper ranges of spontaneous-abortion risk, the cumulative risks of spontaneous abortion from the primary and sensitivity analyses were within the expected risk range.
Study limitations included the lack of a control group of unvaccinated pregnant women, the homogeneity of the study participants and the reliance on self-reported data.
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The researchers concluded that despite these limitations, the findings again show no safety signal consistent with an increased risk for spontaneous abortion associated with mRNA Covid-19 vaccines.

“These findings add to the accumulating evidence about the safety of mRNA Covid-19 vaccination in pregnancy,” they wrote.

  1. Further analysis of data from the CDC’s v-safe Covid-19 vaccine pregnancy registry showed no increased risk for first- or second-trimester miscarriage associated with the mRNA vaccine.

  2. The cumulative risk of spontaneous abortion from 6 to less than 20 weeks gestation among women who received the Pfizer-BioNTech or Moderna vaccines was around 14% in the primary analysis and 13% in an age-standardized analysis.

Salynn Boyles, Contributing Writer, BreakingMED™

This research was funded by the U.S. Centers for Disease Control and Prevention.

The researchers reported no relevant disclosures related to this study.

Cat ID: 125

Topic ID: 79,125,933,125,926,191,41,192,927,151,928,925,934