Early Fc-effector antibody signatures impact COVID-19 disease trajectory.

Why do some individuals develop mild COVID-19 while others progress to severe disease remains a central challenge in SARS-CoV-2 immunology. In this study, we leveraged the BACO Cohort - a unique historical cohort of immunologically naïve, hospitalized COVID-19 patients from the first pandemic wave - to investigate early immune determinants of clinical disease trajectories. Integrating bulk RNA-seq, Olink proteomics, and systems serology, we identified two fundamentally distinct immune trajectories according to disease phenotypes. Severe patients exhibited upregulation of proinflammatory genes and monocyte-associated transcripts, alongside downregulation of genes related to T cell responses and immune signaling. Notably, an upregulation of inhibitory Fc-receptor-associated gene was also found in severe cases. In contrast, mild cases showed coordinated lymphoid activation and limited inflammation. Building on these findings, we performed a functional profiling of Fc-effector activity in the polyclonal serum of the patients and found that monocyte-mediated phagocytosis was a common feature of mild disease. Interestingly, this response was mainly driven by rapid induction of S1-specific antibodies. Conversely, severe patients tended to generate higher levels of S2-biased antibodies early after infection with poor Fc-effector functionality. Together, these findings demonstrate that early S1-directed, Fc-competent humoral immunity is a key determinant of favorable COVID-19 outcomes, while delayed functional maturation and early S2 bias characterized severe disease in the BACO cohort.