mRNA From COVID-19 Vaccine Is Not Transferred Through Mother’s Breast Milk

COVID-19 vaccines, pregnancy and lactation

Mothers and expectant mothers are always concerned with doing what is best for their child. As a result, motherhood requires making many difficult decisions. Add a global pandemic to the mix and these decisions become harder still.

The first COVID-19 vaccines to receive authorization for human use in 2020 underwent the clinical trial procedures that are standard for any investigational drug or preventative. This standard procedure prohibits the inclusion of pregnant or breastfeeding individuals, the reasons for which are many, as explained by Dr. Stephanie Gaw, assistant professor in obstetrics, gynecology and reproductive sciences at UCSF: "There is a desire to 'protect' mothers and their fetuses/infants from potential harm, and the assumption that they would be reluctant to participate."

"Other reasons include increased cost and complexity of the study, as the research team would need to follow pregnancy and infant-specific outcomes, and thus have obstetric and pediatric staff on board. In addition, follow-up would need to be extended longer than many studies would like (after pregnancy and the infant for a period of time)," added Gaw.

COVID-19, Vaccines and the Menstrual Cycle. Credit: LabTube.

Gaw argues that while it may seem sensible to simplify trials in this way, it actually causes harm. As the COVID-19 vaccination schemes commenced across the world and pregnant or lactating women became eligible for immunization, they faced a tough decision: take a potentially life-saving vaccine in the complete absence of data, or risk becoming infected with SARS-CoV-2.

The World Health Organization (WHO) currently recommends that pregnant or breastfeeding women may receive the vaccine if the benefits of vaccination outweigh the potential risks. Dr. Soumya Swaminathan, chief scientist at the World Health Organization, recently reassured that there is no risk here because none of the vaccines that are being used contain live virus: "There's no risk of transmission through the breast milk. In fact, the antibodies that the mother has can go through the breast milk to the babe and may only serve to protect the baby a little."

Nonetheless, some pregnant and lactating women have decided against immunization, citing the lack of available data as their reasoning. Some mothers that have chosen to receive a COVID-19 vaccine are providing research samples for the scientific community, which helps us to learn more about the effects of the vaccine in these populations.

Analyzing breast milk after mRNA vaccination

Gaw is a senior author on a new study published in JAMA Pediatrics that analyzed the breastmilk of lactating women that had received an mRNA-based COVID-19 vaccine. There are a number of medications that can be passed to the baby through the mother's breast milk, such as some antimicrobial drugs or antidepressants. When deciding to take such medications, the mother may have to temporarily pause or completely stop breastfeeding.

"We suspected that there would be minimal vaccine transmitted to the breastmilk, due to the rapid degradation of mRNA in the body. We wanted to work quickly to provide scientific evidence of vaccine safety for the millions of mothers in the world that were considering the vaccines," Gaw told Technology Networks.  

The researchers recruited a total of seven women from the COVID-19 Vaccine in Pregnancy and Lactation (COVIPAL) study, collecting milk samples from 4–48 hours after administration of either the Pfizer–BioNTech vaccine, BNT162b2 (n = 5) or the Moderna vaccine, mRNA-1273 (n = 2). "We designed a sensitive quantitative RT-PCR method to detect vaccine-specific mRNA in clinical samples," Gaw said. "The lower detection limit of our assay was 0.195 pg and 1.5 pg per 200 μ liter well, for the BNT162b and mRNA-1273 vaccines, respectively."

None of the samples showed a detectable level of vaccine mRNA in the breastmilk. "The baby is unlikely to be affected by the vaccine, and there is no known reason to stop breastfeeding after vaccination," Gaw added. Furthermore, the authors note that any residual mRNA below the level of detection in the assay would undergo degradation via the gastrointestinal system, again reducing any chance of infant exposure.

The study authors acknowledge the small sample used is as a limitation to the work, however Gaw anticipates that similar, larger-scale studies will be conducted by other groups to confirm the findings. "We always need more, well-designed studies on COVID-19 and female health. While we have learned a lot in the past year, we still are awaiting robust data on the impact of infection and vaccination early in pregnancy, and the effect on longer term outcomes in children," Gaw concluded.

Stephanie Gaw was speaking with Molly Campbell, Science Writer for Technology Networks.

Reference: Golan Y, Prahl M, Cassidy A, et al. Evaluation of messenger RNA from COVID-19 BTN162b2 and mRNA-1273 vaccines in human milk. JAMA Pediatrics. 2021. doi: 10.1001/jamapediatrics.2021.1929.