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Feature FRANCISCO SECO/AP/SHUTTERSTOCK FRANCISCO A mother breastfeeds her newborn at a hospital in Belgium. olly Siegel had long awaited a COVID-19 vaccine. As an obstetrician at Massachusetts General Hospital in Boston, she BREASTFEEDING AND regularly saw pregnant people with COVID-19, and knew that the vaccine was the best way to pro- tect herself, her family and others Min her workplace. But with a seven-month-old COVID VACCINES: baby at home who was still breastfeeding, she felt hesitant. Understandably so. Following established norms for clinical trials, pregnant and breast- WHAT THE DATA SAY feeding people were not included in any of the trials for COVID-19 vaccines. So, as health Early studies suggest that vaccines are safe and systems around the world began to vaccinate eligible adults, scores of lactating people were that antibodies transfer in milk. But do they left to make their decision in the dark. “I certainly was frustrated that there weren’t protect the baby? By Shannon Hall studies on the vaccine in pregnant and lactating women — that as a group, they were excluded from the research,” Siegel says. “It made it really hard to know, as both the patient and the pro- vider, how to think about the vaccine.” Still, Siegel could not see any plausible risk to her breast milk (she knew that COVID-19 vac- cines contain no live virus, for instance), and focused on the benefit of protecting herself 492 | Nature | Vol 594 | 24 June 2021 ©2021 Spri nger Nature Li mited. All rights reserved. ©2021 Spri nger Nature Li mited. All rights reserved. and everyone around her. So she got the shot. question — as Siegel asked herself — was whether Moderna vaccine, and found no trace of the Then, she donated samples of her breast milk a COVID-19 vaccine would harm a nursing mRNA in either case. (The group is now scour- to researchers who would analyse its contents infant. After all, some medications are not ing a larger number of milk samples for differ- in one of the first such studies. recommended during lactation because they ent components of the vaccine, and expanding Now, thanks to Siegel and other participants, pass through breast milk to infants. Nursing their study to include all the available COVID-19 scientists are beginning to understand the mothers are advised against taking high doses vaccines in the United States.) effects of COVID-19 vaccines on breast milk, of aspirin, for example; even after low doses, and their preliminary results should come as mothers are warned to monitor the infant for Liquid gold welcome news to the more than 100 million signs of bruising and bleeding. Some vaccines There is one type of particle that scientists are lactating people across the world. are off limits, too. The US Centers for Disease eager to see in breast milk following a vaccine: Scientists have so far looked only at the vac- Control and Prevention (CDC) advises nursing COVID-19 antibodies. cines made by Pfizer–BioNTech and Moderna, mothers against receiving the yellow-fever vac- Researchers have long known that newborn and have not detected the vaccines in breast cine, which involves a live, weakened form of babies don’t effectively produce antibodies milk. What they have found are antibodies, pro- the virus, on the off-chance that an infection against harmful bacteria and viruses; and it can duced by mothers in response to inoculations, passes to the infant. take three to six months for this kind of pro- to the coronavirus SARS-CoV-2. Because of such cases, some pharmacists tection to kick in. To help in those early days, a “We’re really happy to have something good and vaccine administrators have been urging mother’s breast milk overflows with antibodies to hang our hat on,” says Stephanie Gaw, a per- nursing mothers to discard their breast milk capable of staving off potential threats. inatologist at the University of California, San after they are vaccinated. “It’s specifically designed by the mother, and Francisco. “The studies are small, they’re still by Mother Nature, to provide the child with early, but very positive.” Now, researchers want “There seems to be the child’s first vaccine,” says Hedvig Nordeng to know whether those antibodies can provide at the University of Oslo, who specializes in babies with at least partial protection against an awful amount of medication use and safety in pregnancy and COVID-19. misinformation out lactation. “Breast milk by itself is more than nutrition, breast milk is medicine.” Vaccine questions there on all levels.” In the mother, immune cells called B lym- Throughout the pandemic, pregnant people phocytes (or B cells) constantly produce and new mothers have been faced with a slew of antibodies. Then, once lactation begins, the concerns and questions about the coronavirus. “I think that clearly shows ignorance mammary glands send out a chemical signal One trend that became clear early on is that and a lack of understanding,” says Kirsi that draws these B cells to the breast — where pregnant people diagnosed with COVID-19 are Jarvinen-Seppo, an immunologist at the Univer- they park in the glands and produce thousands more likely to be hospitalized than are those sity of Rochester Medical Center in Rochester, of antibodies per second, ready to move into of the same age who are not pregnant. That New York. “There seems to be an awful amount the breast milk in huge quantities. But unlike could be because the body is already working of misinformation out there on all levels.” molecules from medications, coffee and alco- hard — the growing uterus pushes upwards, Unlike the yellow-fever vaccine, COVID-19 holic drinks, which are so small they can pass reducing lung capacity, and the immune sys- vaccines do not carry a risk of igniting an active into the breast milk on their own (although tem is suppressed so as not to harm the baby. infection. In addition, COVID-19 vaccines are at diluted levels), antibodies are too large to Those factors don’t disappear the day a baby extremely unlikely to cross into breast milk. do so. Instead, receptors on the surface of the is born. As such, some obstetricians suspect The fragile messenger RNA used in the Pfizer– milk ducts grab the antibodies and package that lactating individuals are also susceptible BioNTech and Moderna vaccines, for exam- them in protective, fluid-filled bubbles that to severe COVID-19. ple, is designed to break down so quickly that allow them to pass safely through the milk- That conclusion might encourage breast- it should never leave the cells where it was duct cells and into the milk on the other side4. feeding mothers to get vaccinated, but scien- injected — let alone get into the bloodstream “This process is so magical,” says Galit Alter, tists weren’t sure how they would respond to and then the breast. In fact, researchers don’t an immunologist and virologist at Harvard the vaccines, because little is known about the expect that any of the current vaccines will be Medical School in Boston, who worked on period of lactation. excreted into breast milk. Gray’s study. So Kathryn Gray, a maternal–fetal medicine To that end, the World Health Organization What happens once antibodies reach the specialist at Brigham and Women’s Hospital recommends that mothers continue to breast- baby, however, is more mysterious. Antibodies in Boston, Massachusetts, and her colleagues feed after vaccination. In addition, the CDC and in the breast milk do not make it into a baby’s decided to test how well the Pfizer–BioNTech the UK Joint Committee on Vaccination and bloodstream, but coat the mouth, throat and Moderna vaccines work in this group. They Immunisation issued statements shortly after and gut before they’re ultimately digested4. recruited 131 participants who were about to the first vaccines were authorized in both coun- Nonetheless, these antibodies seem to provide receive either vaccine and who were lactating, tries. These noted that no safety concerns had protection. It could be that they work at the pregnant or neither, and found that the lactat- been identified from the available data, so lac- body’s entrances to fend off infection before ing individuals (which included Siegel and 30 tating people could choose to be vaccinated. it takes root. others) generated the same robust antibody “It’s sort of a backwards way of recommend- Not all babies are raised on breast milk, but response as did those who were not lactating1. ing it,” argues Christina Chambers, a paediatri- studies have shown that babies who exclu- In other words: the vaccine is just as beneficial cian at the University of California, San Diego, sively breastfeed for their first six months for breastfeeding mums. and the Rady Children’s Hospital. “The founda- have far fewer middle-ear infections than A second study by Gaw and her team, posted tion is that there’s no reason to avoid it, which those who are breastfed for a shorter time, or on the preprint server medRxiv, agrees2. The is a dilemma.” not at all5. They also have a lower risk of respira- team drew blood from 23 lactating partici- So Gaw and her colleagues ran a safety check. tory-tract infections6. And lactating mothers pants and found that antibodies to SARS-CoV-2 In a small study3, her team looked at breast milk who receive the influenza vaccine (and there- increased after their second dose. samples from six participants up to two days fore transmit those protective antibodies But for many parents, the looming after they received the Pfizer–BioNTech or to their infant through breast milk) provide Nature | Vol 594 | 24 June 2021 | 493 ©2021 Spri nger Nature Li mited.

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