All experiments involving mice were conducted following protocols approved by the University of Colorado Institutional Animal Care and Use Committee (IACUC) according to guidelines provided by the Association for Assessment and Accreditation of Laboratory Animal Care. C57BL/6 mice were obtained from the Jackson Laboratory and were subsequently bred in specific-pathogen-free facilities at the University of Colorado Anschutz Medical Campus. Experiments were performed in 6–12-week-old female mice. Mice were immunized via tail-vein injection with 100 μg or 200 μg, of SARS-CoV-2 spike RBD protein plus adjuvant. SARS-CoV-2 RBD protein (Wuhan-Hu-1; GenBank: MT380724.1) was expressed by transfection of Expi293 cells with a His-tagged vector (a gift from F. Krammer, Icahn School of Medicine at Mount Sinai, New York, NY)(11 (link)) and subsequently purified from cell culture supernatants by the University of Colorado Cell Technologies Shared Resource. Immunizations were adjuvanted with 40 μg poly(I:C) (Invivogen), and 40 μg anti-CD40 (clone FGK4.5, BioXCell). Vaccines were made immediately prior to immunization and injected in a total volume of 200 μl.