The emergence of SARS-CoV-2 variants with adaptive mutations necessitates the development of broad-spectrum antivirals. This study investigated two hetero-bivalent nanobodies (Nbs), aRBD-2-5 and aRBD-2-7, previously shown to neutralize wild-type SARS-CoV-2. Crystal structures revealed that aRBD-2 binds to conserved RBD residues, maintaining binding to various variants including Omicron. Fusion with aRBD-5 or aRBD-7 enhanced binding affinity. Both aRBD-2-5-Fc and aRBD-2-7-Fc potently neutralized multiple variants *in vitro*. aRBD-2-5-Fc demonstrated prophylactic and therapeutic protection against Omicron BA.1 in hamsters, suggesting its potential for COVID-19 prevention and treatment.

Huan Ma, Xinghai Zhang, Peiyi Zheng, Peter H. Dube, Weihong Zeng, Shaohong Chen, Qingyu Cheng, Yunru Yang, Yan Wu, Junhui Zhou, Xiaowen Hu, Yan Xiang, Huajun Zhang, Sandra Chiu, Tengchuan Jin