We corroborated our results using the MPro antigen.18Milk samples from COVID-19 infected and recovered donors still showed significantly higher reactivity to the MPro antigen than the prepandemic samples (online supplemental figure S5). (Igs)IgA, IgG and IgMin breast milk samples from patients with COVID-19. == Results == All breast milk samples showed negative results for presence of SARS-CoV-2 RNA. We observed high intraindividual and interindividual variability in the antibody response to the receptor-binding domain of the SARS-CoV-2 spike protein for each of the three isotypes IgA, IgM and IgG. Main Protease (MPro) domain antibodies were also detected in milk. 82.9% (58 of 70) of milk samples were positive for at least one of the three antibody isotypes, with 52.9% of these positive for all three Igs. Positivity rate for IgA was relatively stable over time (65.2%87.5%), whereas it raised continuously for IgG (from 47.8% for the first 10 days to 87.5% from day 41 up to day 206 post-PCR confirmation). == Conclusions == Our study confirms the safety of breast feeding and highlights the relevance of virus-specific SARS-CoV-2 antibody transfer. This study provides crucial data to support official breastfeeding recommendations based on scientific evidence. Trial registration numberNCT04768244. Keywords:COVID-19, microbiology, neonatology, global health ARS-CoV-2 antibodies but not RNA are found in breast milk from COVID-19 infected mothers. == What is already known on this topic? == Breast feeding provides optimal nutrition in infants. Data are conflicting on whether SARS-CoV-2 is present in breast milk of infected mothers. Breast milk of infected mothers contains antibodies to SARS-CoV-2, especially IgA. == What this study adds? == SARS-CoV-2 RNA was not detected in any of the breast milk samples from our study. There is high intra- and inter-individual variability in the antibody response against the receptor-binding domain of the SARS-CoV-2 spike protein for the three antibody isotypes (IgA, IgM and IgG) delta-Valerobetaine and also against nonstructural proteins, like MainProtease (MPro). Rabbit Polyclonal to Transglutaminase 2 Most of the breast milk samples (82.9%) had antibodies after SARS-CoV-2 infection for at least one of the three isotypes, with 52.9% of these positive for all three immunoglobulins. == Introduction == Breast feeding is considered the gold standard for infant feeding and is of crucial importance in influencing both infant growth and development. Epidemiological studies have demonstrated that breast feeding decreases risk of infections in infants.14Due to its beneficial effects, international organisations including the WHO recommend exclusive breast feeding for the first 6 months of life, and continuing breast feeding while complementary foods are introduced until 2 years of age or beyond.5 The COVID-19 global pandemic caused by SARS-CoV-2 has increased concerns about potential mother-to-infant transmission, including via breast feeding. While some studies reported the presence of SARS-CoV-2 in breast milk,6 7although its potential for infection delta-Valerobetaine is unclear,8other studies found no presence of the virus.911In general, these studies showed several limitations, with the most relevant being the lack of targeted and validated protocols for viral detection in milk matrix. Furthermore, a strong antibody response is induced after maternal SARS-CoV-2 infection, with higher presence of neutralising secretory IgA in breast milk.7 12 13However, several questions remain unanswered, including a specific and reliable method to detect SARS-CoV-2 in human milk, the extent of the response, the persistence of maternal antibodies in milk and their potential protective role in infants. Under this scenario, our main objectives were (1) to provide a delta-Valerobetaine specific and reliable detection method for SARS-CoV-2 in breast milk; and (2) to determine the levels of reactive IgA, IgG and IgM antibodies against structural and non-structural SARS-CoV-2 proteins in breast milk collected during the COVID-19 pandemic. == Materials and methods == == Study population == This is a prospective, observational, longitudinal and multicentre study in motherinfant pairs with confirmed SARS-CoV-2 infection. Participants were recruited from seven health centres from different provinces in Spain (Valencia, Barcelona, Granada and Zaragoza). The recruitment period was from April to December 2020. Participants were pregnant women intending to breast feed and nursing women with positive PCR for SARS-CoV-2 on nasopharyngeal swabs or presence of SARS-CoV-2 antibodies in serum determined in hospitals. Women were excluded when COVID-19 symptomatology required specific treatment and/or hospitalisation in intensive care units. Exclusion criteria included women unable to breast feed due to severe symptomatology that required intensive care unit and/or mothers need for drugs with potential adverse effects on the infant and/or impossibility to obtain milk. All participants received oral and written information about the study.