CCL2 and infection: As monocytes and macrophages are sentinel cells that sense invasive infection and release the potent inflammatory mediators (52), we speculate that viral replication likely occurred early after Omicron challenge, which led to the promotion of MCP1 and IL-8 production by the existing S-IgG as well as proinflammatory monocyte/macrophage recruitment and accumulation prior to viral clearance.