Our study shows a significant reduction of Anti-SARS-CoV-2 IgA and IgG immunoglobulin in BM from vaccinated women after pasteurization. Nevertheless, a high percentage of these antibodies (between 70–80% of antibodies depending on IgA or IgG) remained in DHM after the pasteurization. Our results suggest that DHM could confer a potential protection against SARS-CoV-2 should OMM be unavailable. However, the functionality of the remaining immunoglobulins after pasteurization is yet unknown.
Breastfeeding and its potential impact on neonatal health is especially relevant in the context of prematurity one of the leading causes of neonatal mortality worldwide (18, 19). As mothers with preterm birth often have difficulties in breastfeeding (5), most neonatal services are opting for the administration of donor human milk (27) due to its proven benefit protecting against necrotizing enterocolitis (7, 20). While Holder pasteurization (62.5ºC, 30 min) ensures the inactivation of non-heat-resistant viruses, including coronaviruses, if present (21), it also causes the loss of some of the biological, structural and functional properties of fresh milk (9, 17, 22) as it triggers cellular (including B and T lymphocytes) and bacterial destruction, as well as partial or total alteration of the structure and function of some of its components. Thus, to evaluate the effect of pasteurization on the potential protective capacity of breastmilk against viral infection will be especially relevant for its prevention in these especially vulnerable infants. Our results agree with Peila et al. (17), which showed the great impact of Holder pasteurization on the antibodies’ levels. This effect has been also previously observed in total IgA, IgM, cytokines, growth factors and other components (17, 23, 24).
Upon maternal infection with SARS-CoV-2, a rapid and strong antibody response is induced with subsequent accumulation of substantial amounts of specific neutralizing secretory IgA (sIgA) in BM (25, 26). Other studies have also reported the presence of specific antibodies in milk (27, 28). Favara et al. confirmed strongly neutralizing IgA and IgG antibodies against multiple SARS-CoV-2 antigens in BM and serum at 2.5 and 6.5 months post-infection (27).
High levels of specific antibodies after immunization against other viruses, like influenza virus, have been shown (29). Indeed, first studies showed the existence of anti-SARS-CoV-2 antibodies in BM of women vaccinated with mRNA vaccines (11, 30). Furthermore, a study conducted by our research group showed that HM from vaccinated women contained RBD-specific IgA and IgG, with levels increasing significantly after the second dose and decreasing after 3 weeks (14). Interestingly, IgG levels in vaccinated women were significantly higher than those observed in the milk of COVID-19 infected and/or recovered women. These results are in agreement with previous data showing levels of antibodies against SARS-CoV-2 in milk from vaccinated women (14). These results suggested the efficiency of vaccines to generate an IgG response (13). The presence of antibodies highlights the potential ability of BM to protect infants against coronavirus infection and underscores the essential role of breastfeeding also in the context of pandemics.
However, the neutralizing capacity of anti-SARS-CoV-2 antibodies in BM hasn’t been sufficiently explored. In a systematic review, Low et al., concluded that fifty percent of mothers with COVID-19 harbored antibodies in milk capable of neutralizing SARS-CoV-2 infectivity in vitro (31). They suggested that pasteurization of BM would not decrease SARS-CoV-2 antibody titers but its neutralizing capacity (31).
One of the limitations of our study is the lack of determination of the SARS-CoV-2 neutralizing capacity of the antibodies detected after pasteurization. Other limitations could include the low sample size and the lack of information on virus-specific antibodies in blood from neonates. As proposed in the literature, breastfeeding offers protection and passive immunity to infants, but it remains to be investigated whether this effect persists after pasteurization. However, our study sheds light on the presence of antibodies in HDM after pasteurization and ensures future research in the field. There are still many open questions including when SARS-CoV-2 antibodies are produced after maternal vaccination, when they can be detected in BM and how long they persist. Indeed, the actual effectiveness of these antibodies in milk is not really known and it remains to be investigated whether the antibodies are well absorbed in the intestine of infants and pass into the bloodstream and exert a protective function.