SARS-CoV-2 and HCoV IgG Antibodies in the Breast Milk of a Postpartum SARS-CoV-2 Patient Following Bamlanivimab Administration: A Case Report

Breast milk can provide passive immunity to infants, serving as a valuable source of maternal antibodies while remaining a non-invasive sample for investigating maternal immune responses. To date, no studies have evaluated SARS-CoV-2 and potentially cross-reactive HCoV antibodies in breast milk following bamlanivimab administration. A 36-year-old postpartum female was PCR-positive for SARS-CoV-2 four days post-delivery. Bamlanivimab was administered intravenously two days later. Breast milk was collected before bamlanivimab infusion, daily for two weeks post-infusion, then weekly until 102 days post-infusion. Mother and infant sera were collected only at 102 days post-infusion. All milk and serum samples were tested for IgG antibodies against SARS-CoV-2 and HCoV. We observed two distinct SARS-CoV-2 antibody peaks at days 3 and 29 post-infusion, likely representing bamlanivimab transfer and the post-infection antibody response. Beta-HCoV antibodies showed two peaks at days 6 and 29, potentially representing backboosted beta-HCoV responses and/or antibody cross-reactivity with SARS-CoV-2. Infant seropositivity for SARS-CoV-2 102 days post-infusion may represent antibodies from passive transfer via breastfeeding or a subclinical infection. This case highlights the value of breast milk as a non-invasive and repeatable sample to help understand maternal immune responses post-infection, exogenous antibody infusion, and passive antibody transfer during breastfeeding, which can provide insights into maternal–infant health research.