Figure 1.

Mouse immunization procedures and viral challenge schedules

(A) Eight groups of K18-hACE2 (B6) background) mice were immunized with the Delta-RBD protein, Omicron-S protein, or PBS control via intramuscular (i.m.), i.m.-intranasal (i.n.), or i.n. route for three doses at a 3-week interval. Aluminum (Alum) plus monophosphoryl lipid A (MPL) and Poly(I:C) adjuvants were used for i.m. and i.n. routes, respectively. Sera collected from the last immunization were tested for specific IgG antibodies (Abs), subtype Abs, and neutralizing Abs against SARS-CoV-2 original strain and multiple variants. Immunized mice were then challenged with the SARS-CoV-2 Delta variant, and evaluated for protective efficacy via the investigation of survival and weight changes.

(B) Eight groups of B6 mice were immunized as described above, and detected for mucosal IgA Abs from collected bronchoalveolar lavage (BAL) fluid.