C9 and infection: As previously observed for structures of infection- and vaccine-induced human mAb Fabs bound to SARS-2 spike trimers (22, 25, 44, 45), the rabbit mAb Fab cryo-EM structures showed different modes of RBD recognition: M8b-A10, M8b-B8, and M8b-C9 Fabs bound to RBDs only when they were in an “up” conformation (Fig. 4A–C), and M8b-C10 Fab recognized both “up” and “down” RBDs (Fig. 4D).