a, Cartoon depiction of engineered VH binders generated by linking VH domains via Fc-fusion or a 20-amino-acid Gly-Ser linker. b–d, BLI traces of lead VH binders VH-Fc B01 (b), VH2 A01-B01 (c) and VH3 B01 (d) against RBD (upper panel) or Secto (lower panel). e, Sequential BLI binding experiments measuring binding of ACE2-Fc to Secto pre-blocked with our VH binders, showing that multivalent VH binders can block ACE2-Fc binding to Secto. f, Competition serology ELISA with convalescent patient sera, showing that VH-Fc binders can compete with patient antibodies. P1 to P9 represent sera from individuals with a history of previous SARS-CoV-2 infection. C1 and C2 are sera collected from two donors before the SARS-CoV-2 outbreak. Individual data points represent technical replicates (n = 2) from the serum of the same individual and are shown as black circles.