Fig. 5: Protective efficacy of mutI-tri-RBD in hACE2-transgenic mice against challenge with the SARS-CoV-2 prototpye, Delta, and Beta strains, respectively.

a The mice were immunized with two doses of mutI-tri-RBD with 2 µg/dose on Day 0 and Day 21, or injected with two doses of physiological saline as a control. On day 7 after the complete immunization, the sera from the caudal vein of the immunized mice were collected. The titers of pseudo-virus neutralizing antibodies in the sera of the immunized mice against the prototype, Delta and Beta SARS-CoV-2 strains, respectively, were measured for the vaccine groups and compared with those in the corresponding saline-treated groups. Data are presented as means ± SEM. P values were calculated with Student’s t-test. **P < 0.01, ***P < 0.001, ****P < 0.0001. b On day 14 or day 19 after the complete immunization, the mice were challenged with the prototype, Delta and Beta live SARS-CoV-2 viruses (1.5 × 10⁵ TCID₅₀), respectively, and correspondingly control groups were challenged with saline (15 µL) as blank controls. The changes in the body weight of the mice were recorded during virus challenge experiments. Data are presented as means ± SEM. c The lung tissues were collected at 5 or 6 days after virus challenge. The viral load was measured by the copies of N, ORF1ab, and S genes. The viral load less than the detectable limit (<500 copies/μL) was set to half the value of the limit, i.e., 250 copies/mL. Data are presented as mean ± SEM. P values were calculated by using one-way ANOVA followed by Sidak’s multiple comparison test. ****P < 0.0001. d Histopathological examinations of the lung tissues of the mice. Scale bars, 200 μm (20×).