ARS-CoV-2 was first detected in December 2019, leading to a pandemic with an estimated 5-6% mortality rate 1. Akin to SARS-CoV-1, the causative agent of the 2003 SARS outbreak, this is an enveloped betacoronavirus with protrusions of large trimeric 'spike' proteins. Receptor binding domains (RBDs) located at the tips of these spikes facilitate host cell entry via interaction with angiotensin-converting enzyme 2 (ACE2) 2. Spikes are type I transmembrane glycoproteins, formed from a single polypeptide, which transitions into a post-fusion state via cleavage into S1 (N-terminal) and S2 (C-terminal) chains following receptor binding or trypsin treatment 3. In the pre-fusion state, the apical RBD (~22 kDa) is folded down, enshrouded by the N-terminal domain (NTD) of the spike so that the receptor binding site is inaccessible until, it is assumed, an RBD stochastically swings upwards to present the ACE2 binding site 4-7. ACE2 interaction locks the RBD in the 'up' conformation, which drives conversion to the post-fusion form where the S2 subunit engages the host membrane while dispensing with S1 4,5. Neutralizing human monoclonal antibodies (mAbs) that recognize the ACE2 receptor binding site for SARS-CoV-1 and SARS-CoV-2 are generally not cross-reactive between the two viruses and are susceptible to escape mutation 8-12. Indeed, a natural mutation (Y495N) has already been identified at this site (GISAID 13 : accession ID: EPI_ISL_429783 Wienecke-Baldacchino et al.). By contrast, the CR3022 antibody (derived from a SARS-CoV-1-infected patient) cross-reacts strongly with SARS-CoV-2 (see Methods and Fig. 1) and has been shown to recognize a cryptic, conserved footprint on the RBD distinct from the binding epitope of