Antibody Transfer via Breastmilk After SARS-CoV-2 Infection

Antibody transfer via breastmilk represents an evolutionary strategy to boost immunity in early life. Although SARS-CoV-2-specific antibodies have been observed in the breastmilk of mothers with COVID-19, the functional quality of these antibodies remains unclear. Here, we applied systems serology to characterize SARS-CoV-2-specific antibodies in maternal serum and breastmilk to define the mechanism of antibody transfer into breastmilk. Distinct SARS-CoV-2-specific antibody responses were observed in the serum and breastmilk of lactating individuals previously infected with SARS-CoV-2, with a dominant transfer of both IgA and IgM into breastmilk. A spike-specific functional IgG were present in breastmilk, breastmilk IgGs were functionally attenuated. Breastmilk antibodies were less inflammatory than maternal serum antibodies, pointing to a sieve offunctional antibodies to breastmilk. These data highlight the preferential breastmilk transfer of SARS-CoV-2-specific IgA and IgM to neonates, accompanied by selected subpopulations of IgG, positioned to create a non-pathologic, but highly protective barrier against COVID-19 disease. Funding Statement: This work was supported by by NHLBI (grants K08HL1469630-02 and K08HL146963-02S1, to KJG), the National Science Foundation Graduate Research Fellowship under Grant No. (#1745302, to KMP), the NIH (U19 AI142790, U19 AI135995, R37AI080289), the Massachusetts Consortium on Pathogen Readiness (MassCPR). Declaration of Interests: G.A. is the founder of Seromyx. K.J.G. has consulted for BillionToOne, Illumina, and Aetion. INCOMPLETE Ethics Approval Statement: This study was approved by the MGH-BWH Institutional Review Board and the BIDMC Institutional Review Board.