a, Schematic showing hamster study design. b, Impact of RBD-Exo on viral genomic RNA (gRNA) in BAL fluid at 7 d post challenge. c, Impact of RBD-Exo on viral gRNA in oral swabs at the indicated time points. d, RBD-specific binding antibody from hamster serum at week 2 (1 d before challenge), 4 d and 7 d after viral challenge as detected by ELISA. e,f, H&E (e) and Masson’s trichrome (f) images of lung tissues from hamsters at 7 d post challenge. Scale bars, 500 μm (top), 100 μm (bottom). g, Quantitation of lung fibrosis of challenged hamsters by Ashcroft scoring performed blindly. Throughout, each dot stands for data from one animal, n = 5. Data are mean ± s.d., P values calculated by one-way ANOVA with Bonferroni correction. All replicates are biological. The schematic in a was created with BioRender.com.