PROS1 and infection: Human single-domain antibodies specific to the RBD fragment or no-RBD S protein of SARS-CoV-2 have been developed, being able to maintain similar advantages of nanobodies (such as small size and nanomolar binding affinities), as well as neutralizing activity and/or protective efficacy against infection of SARS-CoV-2 original strain and variants [120–123].