Figure 2. Vaccination-induced protection against lethal MA-15 infection at different stages.

Post immunization: (A) Neutralizing antibody titers after immunization. Post challenge: (B) daily survival rates, (C) daily weight loss, and the viral loads in the lung on day 1 (D) and day 2 (E).Groups of mice (N=15 per group) were immunized 3 times with yeast expressed RBDs (20, 10, and 10 μg respectively) or 9 μg of S protein for each immunization at 3-week intervals. Mice given TBS/alum were included as controls. The titers of neutralization antibodies were determined on day 50. All vaccinated mice were challenged with 5.6 logs (~ 10X LD₅₀) TCID₅₀/60 μL of MA-15 intranasally (IN). Three challenged mice in each group were euthanized on days 1 and 2 post-challenge, respectively. The remaining mice in each group (N=9) were monitored daily for clinical manifestations (e.g., weight loss), and mortality.