Through direct cloning and expression of the variable heavy chains (VHHs) of HCAbs from the bone marrow of MERS-CoV infected dromedaries, MERS-CoV specific VHHs or nanobodies were produced. VHH-83 binds with exceptionally high affinity to the receptor binding domain of the viral spike protein. In vitro, this VHH efficiently blocks virus entry at picomolar concentrations