Messenger RNA (mRNA) vaccines against COVID-19 were recently approved under an emergency use authorization.1 However, there is a paucity of data regarding vaccine safety in pregnant or lactating individuals who were excluded from phase 3 clinical trials,2,3 and many mothers have declined vaccination or decided to discontinue breastfeeding (temporarily or permanently) due to concern that maternal vaccination may alter human milk. The World Health Organization recommends that breastfeeding individuals be vaccinated and does not advise cessation of breastfeeding following vaccine administration.4,5 The Academy of Breastfeeding Medicine states that there is little plausible risk that vaccine nanoparticles or mRNA would enter breast tissue or be transferred to milk,6 which could theoretically result in priming of infant immune responses that could alter childhood immunity. However, there are no direct data. To address this knowledge gap, we analyzed milk samples to determine if vaccine-related mRNA was detectable in human milk after vaccination.
The institutional review board of the University of California, San Francisco, approved the study. Written informed consent was obtained from all study volunteers in the COVID-19 Vaccine in Pregnancy and Lactation (COVIPAL) cohort study from December 2020 to February 2021. Clinical data were collected by questionnaires. Self-collected milk samples were kept on ice or immediately frozen (at home) until arrival in the laboratory. Samples were collected prior to vaccination and at varied time points up to 48 hours after vaccination. Total RNA was isolated from milk components using the RNeasy Mini Kit (Qiagen). We performed real-time quantitative polymerase chain reaction assay targeting the mRNA used in the COVID-19 mRNA-based vaccines. The BNT162b2 (Pfizer) and mRNA-1273 (Moderna) vaccines were separately inoculated into prevaccination milk samples, which were processed by the same protocols and used as positive controls for this assay (eMethods in the Supplement); prevaccination milk samples were used as negative controls. Based on standard curves, we found that the lower detection limit of our assay was 0.195 pg and 1.5 pg for the BNT162b and mRNA-1273 vaccines, respectively. Because vaccine uptake and mRNA content may differ between milk fractions, we analyzed supernatant and fat separately for all milk samples. Two samples that had sufficient milk cellular material were analyzed separately. A single freeze/thaw cycle of vaccine-inoculated milk samples did not negatively affect mRNA detection compared with fresh samples. Positive controls had higher levels of mRNA-1273 in the fat layer than in the milk supernatant. QuantStudio Software version 1.7.1 (Applied Biosystems) and Prism version 9.1.0 (GraphPad) were used for analyses.
A total of 7 breastfeeding mothers (mean [SD] age, 37.8 [5.8] years) volunteered for this study (Table). Their children ranged in age from 1 month to 3 years. Postvaccination milk samples were collected 4 to 48 hours after administration of the BNT162b2 (n = 5) or mRNA-1273 (n = 2) vaccines. Analysis of 13 human milk samples collected 24 hours after vaccination, including multiple time points (4 to 48 hours) from a single participant, revealed that none of the samples showed detectable levels of vaccine mRNA in any component of the milk (Figure).
Vaccine-associated mRNA was not detected in 13 milk samples collected 4 to 48 hours after vaccination from 7 breastfeeding individuals. These results provide important early evidence to strengthen current recommendations that vaccine-related mRNA is not transferred to the infant and that lactating individuals who receive the COVID-19 mRNA-based vaccine should not stop breastfeeding. In addition, any residual mRNA below the limits of detection in our assay would undergo degradation by the infant gastrointestinal system, further reducing infant exposure. Limitations of this study are the small sample size and few participants who received the mRNA-1273 vaccine. In addition, milk storage conditions may affect mRNA stability. Clinical data from larger populations are needed to better estimate the effect of these vaccines on lactation outcomes.
Accepted for Publication: April 26, 2021.
Published Online: July 6, 2021. doi:10.1001/jamapediatrics.2021.1929
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2021 Golan Y et al. JAMA Pediatrics.
Corresponding Author: Stephanie L. Gaw, MD, PhD, Division of Maternal-Fetal Medicine, Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, 513 Parnassus Ave, HSE16, PO Box 0556, San Francisco, CA 94143 (email@example.com).
Author Contributions: Drs Golan and Gaw had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: All authors.
Acquisition, analysis, or interpretation of data: Golan, Prahl, Cassidy, Lin, Gaw.
Drafting of the manuscript: Golan, Cassidy, Ahituv, Gaw.
Critical revision of the manuscript for important intellectual content: Prahl, Cassidy, Lin, Ahituv, Flaherman, Gaw.
Statistical analysis: Golan, Cassidy, Gaw.
Obtained funding: Golan, Prahl, Ahituv.
Administrative, technical, or material support: Golan, Cassidy, Lin, Ahituv, Flaherman, Gaw.
Study supervision: Prahl, Ahituv, Gaw.
Conflict of Interest Disclosures: Dr Prahl has received grants from the National Institutes of Health and Marino Family Foundation. Dr Golan has received a postdoctoral fellowship from the International Society for Research In Human Milk and Lactation and Human Frontier Science Program. Dr Flaherman has received grants from the Bill and Melinda Gates Foundation, US Centers for Disease Control and Prevention (CDC) Foundation, Robert Wood Johnson Foundation, California Health Care Foundation, and Yellow Chair Foundation. Dr Gaw has received grants from the National Institutes of Health, Bill and Melinda Gates Foundation, CDC Foundation, Robert Wood Johnson Foundation, California Health Care Foundation, and Yellow Chair Foundation. No other disclosures were reported.
Funding/Support: This study was supported by the Marino Family Foundation (Dr Prahl), the National Institutes of Health (grant K23AI127886 to Dr Prahl and grant K08AI141728 to Dr Gaw), the Weizmann Institute of Science–National Postdoctoral Award Program for Advancing Women in Science (Dr Golan), the International Society for Research In Human Milk and Lactation Trainee Bridge Fund (Dr Golan), and the Human Frontier Science Program (Dr Golan).
Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Additional Contributions: We acknowledge the milk donors that volunteered for this study. We thank Kenneth Scott, BS, RPh, (UCSF Health Pharmacy, University of California, San Francisco) and Hannah J. Jang, PhD, RN, PHN, CNL (UCSF School of Nursing, University of California, San Francisco), for voluntarily providing unused vaccine for this study. We also thank Caryl Gay, PhD (Department of Family Health Care Nursing, UCSF School of Nursing, University of California, San Francisco), and Ifeyinwa V. Asiodu, PhD, RN, IBCLC (Department of Family Health Care Nursing, UCSF School of Nursing, University of California, San Francisco), for voluntary assistance with participant questionnaires and support of the study. Contributors were not compensated for their work.