Vaccines against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, were developed at an unprecedented pace. To date, more than 1 billion vaccine doses have been administered worldwide, many of which messenger RNA (mRNA) based.
Clinical trials for both the Pfizer-BioNTech BNT162b2 and Moderna mRNA-1273 vaccines demonstrated the ability to prevent infection and severe disease, leading to emergency use authorization U.S. Food and Drug Administration (FDA).
The American College of Obstetrics and Gynecology and The Society for Maternal-Fetal Medicine have recommended that these mRNA vaccines be made available for lactating women. However, the initial trials excluded breastfeeding women, leading to questions about their safety.
One study of 31 breastfeeding women who received an mRNA vaccine found that >60% reported side effects. Researchers led by Kerri Bertrand and colleagues, from the University of California, San Diego, sought to evaluate a larger sample of vaccinated breastfeeding women and their breastfed children. The research paper is posted to the medRxiv* server.
They found that >85% of participants reported local or systemic symptoms for both the Pfizer-BioNTech and Moderna vaccines following either dose.
The study cohort involved 180 breastfeeding women who had received either one of the mRNA vaccines (Pfizer-BioNTech BNT162b2 and Moderna mRNA-1273 COVID-19 vaccines). These women were enrolled, December 14, 2020, and February 1, 2021, into the Mommy’s Milk Human Milk Research Biorepository at the University of California, San Diego.
While 71.1% of women received the Pfizer vaccine, the other 28.9% received the Moderna brand; most of them were exclusively breastfeeding their babies (the age at enrollment averaged 7.47 months).
The researchers observed that a similar number of women reported symptoms by either of the vaccine brands. Notably, the frequency of specific symptoms did not differ by brand.
However, following dose two, women who received the Moderna brand reported significantly more systemic side effects: chills, muscle/body aches, fever, vomiting. They also observed reports of localized symptoms including pain, redness, swelling, or itching at the injection site than women following dose two of the Pfizer brand.
Although a small proportion of women experienced a reduction in milk supply (8.0% vs. 23.4% for Pfizer and Moderna, respectively, after the second dose), the milk production returned to normal within 72 hours.
“These data are reassuring regarding the safety of vaccination in breastfeeding women and their breastfed children with either of the mRNA COVID-19 vaccines,” wrote the authors.
The team also reported that five women observed a change in the color of milk (blue-green).
This study also noted some behavioral changes in the children; the most common were irritability, poor sleep, significantly more drowsiness reported for children whose mothers received the Pfizer vaccine - all of these outcomes are non-serious. The researchers claimed that this is the first study published on outcomes in breastfed children.
“Because non-replicating vaccines pose no risk for lactating people or their infants, COVID-19 vaccines are also thought to not be a risk to the breastfeeding infant.” National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases (Last Updated March 18, 2021).
While there is a starkly higher percentage of women complaining of symptoms post-vaccination (compared to the previous study by Gary et. al), the researchers reasoned that it could be due to the differences in assessment methods, timing, and the number of symptoms specifically queried.
In short, this study informs about the immediate safety of COVID-19 vaccines in lactating people, the effects of vaccination on the breastfed infant, and the effects on milk production or excretion. Additional studies are underway to evaluate milk composition and antibody status in samples obtained from women participating in the current study, the researchers inform.