A group from Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA, etc. has reported that SARS-CoV-2 specific antibody transfer to breastmilk is dominated by IgA and IgM.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8531199/
Previous studies clearly illustrated the evolution and transfer of Fc-effector function in pregnant women via placenta to their infants, but less is known about the transfer of Fc-effector function across breastmilk.
As expected, mothers infected with SARS-CoV-2 possessed SARS-CoV-2-specific antibodies in serum and breastmilk that were not present in SARS-CoV-2-uninfected specimens. But, interestingly, robust transfer of IgA and IgM was observed in breastmilk, but the transfer of IgG1 was more limited as shown below.
