The vaccine-elicited immunoglobulin profile in milk after COVID-19 mRNA-based vaccination is IgG-dominant and lacks secretory antibodies

The Pfizer/BioNTech and Moderna mRNA-based COVID-19 vaccines are licensed under emergency use authorization, with millions of doses already administered globally [1]. No COVID-19 vaccines are yet under investigation for use in infants or young children. As such, the passive immunity of the antibodies (Abs) provided through milk from a vaccinated person may be one of the only ways to protect this population until pediatric COVID-19 vaccines are licensed. Our early work (as well as an expanded study being published concurrently with this report) examining the milk Ab response after SARS-CoV-2 infection demonstrated that Spike-specific IgA in milk after infection is dominant and highly correlated with a secretory Ab response [2]. Determining if secretory Abs are elicited in milk is critical, as this Ab class is highly stable and resistant to enzymatic degradation in all mucosae - not only in the infant oral/nasal cavity and gut, but in the airways and GI tract as well [3, 4]. Presently, we describe our analysis of the milk Ab response 14 days after completion of an mRNA-based COVID-19 vaccine regimen among 10 individuals. It was evident that unlike the post-infection milk Ab profile, IgG dominates after COVID-19 vaccination. One hundred percent of post-vaccine milk contained significant levels of Spike-specific IgG, with 8/10 samples exhibiting high IgG endpoint titers. Conversely, 6/10 (60%) of post-vaccine samples were positive for Spike specific IgA, with only 1 (10%) exhibiting high IgA endpoint titer. Furthermore, 5/10 (50%) post-vaccine milk samples contained Spike-specific secretory Ab, none of which were found to be high-titer. As our analyses of the immune response in milk to COVID-19 vaccination continues, it will provide a critical opportunity to address huge knowledge gaps, inform the field as to which COVID-19 vaccine, if any, is likely to provide the best milk Ab response, and highlight the need to design improved vaccines with protection of the breastfeeding infant in mind.