Maternal SARS-CoV-2 Antibodies in Mink Kits: Dynamics of Passive Immunity and Implications for Silent Farm-Wide Transmission Following Virus Re-Incursion

Maternal antibodies protect mink kits early in life, but their duration and impact on subsequent SARS-CoV-2 exposure remain unclear. We longitudinally characterized passive immunity in kits born to mothers previously infected with lineage B.1.1.305 on a commercial farm in Greece and related antibody kinetics to a later heterologous virus incursion. Breeder animals showed a 93.3% seropositivity rate after the initial outbreak. On day 20 postpartum, 81.4% of mothers and 69.8% of kits were positive for SARS-CoV-2 nucleocapsid (N)-specific antibodies by enzyme-linked immunosorbent assay (ELISA); a maternal S/P% >160.1% was associated with kit seropositivity at the same time-point (area under the curve 0.985; sensitivity 90%; specificity 100%). Kit seropositivity declined to 2.4% by D*56, consistent with the rapid waning of maternally derived antibodies. On D*87, 97.6% of kits had seroconverted, and 90.0% of mothers were seropositive; Analysis of oropharyngeal swabs by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and next generation sequencing (NGS) confirmed introduction of lineage B.1.177. No clinical signs or excess mortality were observed during this reinfection event. These data demonstrate efficient passive transfer, rapid loss of detectable antibodies by ∼8 weeks, and farm-wide seroconversion after re-exposure to a new lineage. The absence of disease despite widespread seroconversion suggests maternally acquired immunity may have mitigated illness but did not prevent infection. Silent SARS-CoV-2 circulation can therefore occur in partially immune mink farms, with important implications for biosecurity and targeted SARS-CoV-2 surveillance. To our knowledge, this is the first longitudinal study to track the dynamics of maternally derived SARS-CoV-2 antibodies in mink kits and link them to subsequent exposure outcomes.