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Background Since the outbreak of COVID-19, many put their hopes in the rapid development of effective immunizations. For now patient isolation, physical distancing and good hygiene are the sole measures for prevention. Processed breast milk with antibodies against SaRS-CoV-2 may serve as additional protection. We aimed to determine the presence and neutralization capacity of antibodies against SaRS-CoV-2 in breastmilk of mothers who have recovered from COVID-19. Methods This prospective case control study included lactating mothers, recovered from (suspected) COVID-19 and healthy controls. Serum and breastmilk was collected. To assess the presence of antibodies in breastmilk and serum, we used multiple complementary assays, namely ELISA with the SARS-CoV-2 spike protein, SARS-CoV-2 receptor binding domain (RBD) and with the SARS-CoV-2 nucleocapsid (N) protein for IgG and bridging ELISA with the SARS-CoV-2 RBD and N protein for total Ig. To assess the effect of pasteurization breastmilk was exposed to Holder Pasteurization and High Pressure Pasteurization. Results Breastmilk contained antibodies against SARS-CoV-2 using any of the assays in 24 out of 29 (83%) proven cases, in six out of nine (67%) suspected cases and in none of the 13 controls. In vitro neutralization of SARS-CoV-2 clinical isolate virus strain was successful in a subset of serum (13%) and milk samples (26%). Although after pasteurization of the milk SARS-CoV-2 antibodies were detected with both methods of pasteurization, virus neutralizing capacity of those antibodies was only retained with the HPP approach. Conclusion Breastmilk of mothers who recovered from COVID-19 contains significant amounts of IgA against SARS-CoV-2, both before and after pasteurization. Key Points Question Does breastmilk of mothers who have recovered from coronavirus disease 2019 (COVID-19) contain antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)? Findings We provide multiple lines of evidence on the presence of a variety of antibodies against SARS-CoV-2, with no such antibodies present in the controls. These antibodies are capable of neutralizing a clinical isolate of SARS-CoV-2 in vitro. We furthermore show that high pressure pasteurization hardly affects antibody levels and efficacy. Meaning Breastmilk, obtained from mothers who have recovered from COVID-19, may serve as a safe and widely applicable preventive strategy for vulnerable high risk populations

The human body produces a vast variety of circulating immunoglobulins (Igs) to recognize and combat pathogens and other non-self molecular components. In human plasma the most abundant class of Igs is the immunoglobulin G subclass I (IgG1) 1. Through somatic recombination and hypermutation, our bodies can theoretically produce several billions of distinct IgG1 variants 2,3. The theoretically available IgG1 repertoire thereby far exceeds the physical number of memory B cells available 4. The theoretical possibilities are highly suggestive of a vastly complex IgG1 plasma repertoire, but here we show that in all studied individuals, this repertoire is dominated by only a few dozens of clones. Our data indicate that each person’s IgG1 repertoire is distinctly unique, representing a personalized barcode. We sequentially measured IgG1 repertoires of critically ill individuals with hospital-acquired sepsis, revealing the occurrence and disappearance of specific IgG1 clones during the evolution of the disease. We demonstrate here that 1) personalized IgG1 profiling by LC-MS is feasible, 2) each person exhibits a unique serological IgG1 repertoire, 3) this repertoire adapts to changes in physiology, and 4) that individual plasma IgG clones can be de novo sequenced by integrative protein-centric and peptide-centric proteomic approaches. We foresee that the presented mass spectrometric approach will accommodate more rapid development of monoclonal antibody treatments, immediately assessing fully human, matured, and optimized molecules. The potential of repertoires from disease survivors can then be used to prevent disease excesses, as was demonstrated for Ebola 5-7, and is the hope for the current COVID-19 pandemic.