A study found COVID-19 antibodies in the breast milk of moms who had caught COVID-19 or been vaccinated.
It's not yet clear whether the antibodies would protect babies from getting sick.
The study was small. One expert called it "useful" but questioned the methods.
A small lab study found antibodies that fight COVID-19 in the breast milk of mothers who had previously caught the coronavirus or received a COVID-19 vaccine.
The study of 77 people from researchers at the University of Rochester, New York, found that the breast milk of those that previously caught the virus contained more of one type of antibody, IgA, while the breast milk of mothers who had received a COVID-19 vaccine contained higher levels of a more common antibody type, IgG.
It's not yet clear if either antibody type will protect the baby from COVID-19, or how long the antibodies last for. Antibodies are just one part of the immune response.
Professor Richard Tedder, a member of the UK Clinical Virology Network, said in a statement to the Science Media Center that the latest research was "useful" - but that the way the researchers measured antibodies was suited for blood testing rather than breast milk. It was not clear if this method would reliably detect antibodies, he said.
The virus that causes COVID-19 was not detected in any milk sample, according to the authors of the study, which was published in JAMA Pediatrics on Wednesday.
The study is one of the first to compare antibodies in breast milk after infection and vaccination, and measure how these antibody levels change over time in the lab. Breast milk often reflects the immune status of the mother.
To get the results, the researchers recruited 47 health workers due to get a vaccine and 30 people from social media or word of mouth who'd had COVID-19. They were recruited before the highly infectious Delta variant became the most common variant in the US in early July.
Antibody levels in the group that previously had COVID-19 varied "significantly" from person-to-person - 73% of the group had antibodies lasting more than 90 days, while 11% showed "little or no response," the researchers said.
Antibodies were more consistent in the vaccine group, the study authors said. There was generally no difference in antibody response between those vaccinated with Pfizer or Moderna's vaccines, they added.
Participants in the infection group self-collected breast milk samples at the start of the study then at day seven, 10, 28 and 90. The vaccine group gave samples before the first dose, 18 days after the first dose, 18 days after the second dose, and 90 days after the second dose, the study authors said.
The mums were 33 years old in the vaccine group, on average, and 29 years old in the infection group, the study authors said.
The Centers for Disease Control and Prevention recommends pregnant people get vaccinated because of a higher risk of COVID-19 complications, rather than to optimize the benefit of antibodies in breast milk.