The outbreak of COVID-19 has severely impacted global health and economy. Cost-effective therapeutics are urgently needed. Here, we used camelid immunization and proteomics to identify a large repertoire of highly potent neutralizing nanobodies (Nbs) to the SARS-CoV-2 spike protein receptor-binding domain (RBD). We discovered multiple elite Nbs of picomolar to femtomolar affinities that inhibit viral infection at as low as sub-ng/ml concentration, more potent than some of the best human neutralizing antibodies. We then determined a crystal structure of such an elite neutralizing Nb in complex with RBD. Structural proteomics and integrative modeling revealed multiple distinct and non-overlapping epitopes and indicated an array of potential neutralization mechanisms. Structural characterization facilitated the bioengineering of novel multivalent Nb constructs into multi-epitope cocktails that achieved ultrahigh neutralization potency (IC50s as low as 0.058 ng/ml) and may prevent mutational escape. Our Nbs can be rapidly produced in bulk from microbes and resist heat, lyophilization, and aerosolization. These highly promising agents are readily translated into efficient, economic, and convenient therapeutics to help end this once-in-a-century health crisis.

Vaccines are urgently needed to combat the global coronavirus disease 2019 (COVID-19) pandemic, and testing of candidate vaccines in an appropriate non-human primate (NHP) model is a critical step in the process. Infection of African green monkeys (AGM) with a low passage human isolate of SARS-CoV-2 by aerosol or mucosal exposure resulted in mild clinical infection with a transient decrease in lung tidal volume. Imaging with human clinical-