However, the study published recently in the journal JAMA Pediatrics does not imply that breast milk antibodies can provide protection against COVID-19 for nursing children.
The researchers collected samples from 77 mothers 47 in the infected group, 30 in the vaccine group to determine the level of antibodies in breast milk over time.
Mothers who had disease-acquired immunity produced high levels of Immunoglobulin A (IgA) antibodies against the virus in breast milk, while vaccine-acquired immunity produced robust Immunoglobulin G (IgG) antibodies, they said.
According to the researchers, both antibodies provided neutralisation against SARS-CoV-2, the first time such evidence has been discovered for both IgA and IgG antibodies.
A neutralising antibody defends a cell from an infectious particle by preventing any effect it has biologically.
''It's one thing to measure antibody concentrations, but it's another to say that antibodies are functional and can neutralise the SARS-CoV-2 virus,'' said study co-author Bridget Young, an assistant professor at the University of Rochester Medical Center (URMC), US. ''One of the exciting findings in this work is that breast milk from both mothers with COVID-19 infection, and from mothers receiving mRNA vaccination contained these active antibodies that were able to neutralise the virus,'' Young said.
The study represents the longest time period that disease-acquired antibodies have been examined post-illness, and the results showed that these antibodies exist for three months after infection, the researchers said.
For vaccinated mothers, the study found evidence of a mild-to-modest decline in antibodies, on average, three months post-vaccination.
''The trend in breast milk antibodies aligns with what we see in vaccination sera,'' said study co-author Kirsi Jarvinen-Seppo, Chief of Pediatric Allergy and Immunology at URMC. ''After a few months, the antibodies trend downward, but the levels are still significantly above what they were pre-vaccine," Jarvinen-Seppo said.
However, the team, including researchers from New York University, US, noted that it is not yet shown whether these breast milk antibodies can provide protection against COVID-19 for nursing children.
''The study does not imply that children would be protected from illness, and breast milk antibodies may not be a substitute for vaccination for infants and children, once approved,'' Jarvinen Seppo added.