This study shows that not only is natural immunity from a current (or past) Covid infection transferred to the infant via breastmilk, but those parts of it that are transferred are the neutralizing and neutrophill-activating IgA, IgM, and IgG, “suggesting a selection of specific, non-inflammatory antibody subsets in the breastmilk.”
Also, the more sick the mother, the more immunity she provides to her baby.
The authors performed some very complicated analyses on the donated breastmilk, and used a comparison group of lactating mothers who had not had Covid and tested negative.
NOTE: I am wondering if there might be a future for bovine colostrum as a disease preventive, as there is with the flu? See “What About Colostrum?“