Are SARS-CoV-2 Antibodies Detectable in Human Milk After Vaccination Against COVID-19?

Human milk (HM) permits transfer of immunity against infections to infants via bioactive factors. The role of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies in HM is poorly understood [1, 2]. This study evaluated SARS-CoV-2 antibodies in the HM of vaccinated healthcare workers (HCW).

This prospective study of 122 HCWs was performed from February to April 2021 at the Hospital Universitario Nuestra Señora de Candelaria. Immunoglobulin G (IgG) against nucleocapsid protein and IgG, immunoglobulin M (IgM), and immunoglobulin A (IgA) antibodies against spike 1 protein receptor-binding domain against SARS-CoV-2 (anti-SARS-CoV-2 RBD-S1) were analyzed. Unvaccinated breastfeeding mothers without COVID-19 were the control group.The 98 vaccinated participants underwent serum and HM evaluation 14 days after receiving 2 doses of either BNT162b2 mRNA (94%) or mRNA-1273 (6%) coronavirus disease 2019 (COVID-19) vaccines. The mean SARS-CoV-2 RBD-S1 IgG serum concentration was 3379.64 binding antibody units (BAUs)/mL with neutralizing antibody titers >560.9 BAUs/mL. Serum SARS-CoV-2 antibodies in the 24 unvaccinated participants were negative. The HM from vaccinated participants had anti-SARS-CoV-2 RBD-S1 IgG with a mean of 12.19 BAUs/mL compared to 0.02 BAUs/mL (P < .001) in HM from unvaccinated participants. Anti-SARS-CoV-2 S1 IgA was noted in 89% of HM from vaccinated women; no anti-SARS-CoV-2 S1 IgM was detected.A positive correlation was reported between anti-SARS-CoV-2 RBD-S1 IgG in serum and HM (r = 0.36; P < .001). This association was stronger if breastfeeding had been <24 months (r = 0.67; P < .001) vs ≥24 months (r = 0.32; P = 0.19). In subgroup analysis, breastfeeding for >24 months and high serum anti-SARS-CoV-2 RBD-S1 IgG levels predicted high HM IgG levels. This was an independent association in both linear and multiple regression models. Compared with breastfeeding <24 months, lactation >24 months was associated with increased HM anti-SARS-CoV-2 RBD-S1 levels.

This study in breastfeeding HCWs showed that the HM antibody levels were higher in women who had been breastfeeding for >24 months prior to receiving 2 doses of COVID-19 vaccine compared to participants who had been breastfeeding <24 months. Limitations include lack of in vitro plaque reduction neutralization tests which is the gold standard for evaluating SARS-CoV-2 antibody deactivation effectiveness. The study was conducted at a single site and did not assess infant serology or clinical outcome.According to the authors, breastfeeding by vaccinated women during a pandemic when young children are ineligible for vaccination may be encouraged. These results support findings from other studies of vaccines, such as influenza, in which the HM of vaccinated women may confer protection to their infants [3]. The benefits of maternal immunization, including the duration of protection afforded by HM from maternal recipients of mRNA COVID-19 vaccines, are research areas deserving of additional exploration. Additionally, further understanding of the association of the duration of receipt of HM from vaccinated women on infant immune responses would be beneficial in understanding the potential for passive protection through nutrition.