Impact of mRNA-1273 vaccination during early pregnancy

The mRNA coronavirus disease 2019 (COVID-19) vaccines are safe and effective for teenagers and adults. But, there is limited research on the effects of vaccination on fetal development during pregnancy.

Study: No evidence of fetal defects or anti-syncytin-1 antibody induction following COVID-19 mRNA vaccination. Image Credit: rarrarorro/Shutterstock

Previous work has suggested that vaccinated pregnant women can pass on antibodies to their newborn babies. Researchers from Yale University School of Medicine found that vaccination during the first trimester does not produce congenital disabilities or size abnormalities. Additionally, there was no evidence of mRNA COVID-19 vaccines causing infertility.

The findings debunk claims that the mRNA vaccine during pregnancy is unsafe and may encourage pregnant women to get immunized. A preprint version of the study is available on the bioRxiv* server while the article undergoes peer review.

The study

Pregnant mice were given a single intramuscular injection of the Moderna vaccine during early gestation. Fetuses were collected at term but before birth to assess any changes in fetal size or congenital disabilities. The vaccine dose โ€” 2 micrograms for mice weighing at an average of 25 grams โ€” was 50 times over the standard dose given to humans. This was done intentionally to detect any vaccine effects on the developing fetus.

The researchers focus exclusively on early gestation because research suggests the fetus is vulnerable to congenital disabilities when activating the maternal innate immune response.

A separate group of pregnant mice was administered a potent TLR3 agonist that induces the maternal immune system and causes fetal deformities and death. As predicted, this group of litter experienced decreased fetal size and weight. No other congenital disabilities were observed.

In contrast, the litter of mice from vaccinated mothers had no noticeable congenital disabilities. Additionally, there were no changes to fetal length and weight. Next, the researchers measured antibody levels against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the fetus to confirm that vaccine-induced antibodies could cross over to the fetus.

Maternal and fetal serum samples were collected 12 days after vaccination and before birth. Their serum levels confirmed high levels of circulating antibodies against the SARS-CoV-2 spike protein and receptor binding domain compared to mice given a sham injection.

The researchers studied the level of anti-syncytin-1 antibodies in human serum samples collected from unvaccinated and vaccinated adult volunteers from a previous study.

The antibody levels were quantified to determine the risk of infertility after vaccination. Both groups were compared to a separate cohort of patients with systemic lupus erythematosus (SLE). This disease is associated with male and female infertility. There were two patients found with SLE with high anti-syncytin-1 antibody levels. Unvaccinated and vaccinated groups both had low levels of anti-syncytin-1 antibodies compared to the two patients with SLE.

Antibody levels were also assessed before vaccination, a week after receiving the second dose, and a month after receiving the second dose. The vaccinated cohort involved people who received either the Moderna or Pfizer-BioNTech vaccine.

Postvaccination after the second dose had the highest level of antibodies targeting SARS-CoV-2. Peak antibody level was seven days after the second dose.

Anti-syncytin-1 IgG levels were not changed in vaccinated adults 7 and 28 days after vaccination with a second mRNA dose compared to levels before vaccination.

Study limitations

Unlike the mRNA vaccine schedule that requires two doses, the current study only administered one Moderna vaccine dose in pregnant mice. Additionally, the study focused on the potential vaccine-related congenital disabilities and size during the first trimester. The effects of mRNA vaccination during the second and third trimester remains poorly understood.

There was no association found between mRNA vaccination and infertility. However, the researchers acknowledge that the study evaluated a small cohort of 51 subjects and that future studies are needed to validate the claim.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.