he SARS-CoV-2 virus is thought, based on sequence identity, to have crossed from bats to humans in 2019 1 . Similar to SARS-CoV-1 (2002-2003 and MERS-CoV (2012), SARS-CoV-2 presents as a respiratory disease but can progress into internal organs and cause organ failure 2,3 . A recent report from France estimates a fatality rate of 0.7% and a hospitalization rate of 3.6% 4 . Both these rates are much higher in elderly populations 4,5 . Around 33% of those admitted to UK hospitals with COVID-19 have died 6 . Because SARS-CoV-2 also spreads rapidly in the naive human population 7 , the current COVID-19 pandemic has presented an unprecedented challenge to modern human society. Although there is currently no 'cure' or vaccine for the disease, passive immune therapy by transfusing critically ill COVID-19 patients with serum from COVID-19 convalescent individuals has been shown to improve clinical outcomes 8,9 . This would suggest that neutralization of the virus, even at a relatively late stage in the disease, may be a useful COVID-19 therapy.The single-positive-strand RNA genome of SARS-CoV-2, like SARS-CoV, encodes four major structural proteins: spike, envelope, membrane and nucleocapsid. The spike protein comprises an N-terminal (S1) subunit, which contains the roughly 200-residue receptor binding domain (RBD) 10,11 , and a C-terminal subunit (S2), which contains the fusion protein 12 (Fig. 1a). The RBD of SARS-CoV-2 binds more tightly to the extracellular domain of angiotensin-converting enzyme 2 (ACE2) (Fig. 1a) than the homologous SARS-CoV-1 RBD 13 . The higher affinity results from sequence changes in RBD (Fig. 1b) and this has been proposed to underlie the higher transmissibility of SARS-CoV-2 14 . Antibodies raised to the spike protein of SARS-CoV-1 can neutralize the virus both in vitro and in vivo, by binding to the RBD and blocking binding to ACE2 15 . Unfortunately, most of these antibodies do not cross-react with the SARS-CoV-2 RBD 13 . The CR3022 antibody derived from a convalescent SARS-CoV-1 patient is cross-reactive to both SARS-CoV-1 and SARS-CoV-2 RBD (reported apparent K D of 6 nM, ref. 16 ). Two studies have reported crystal structures of CR3022 bound to SARS-CoV-2 RBD and show that the target epitope is distant from the ACE2 binding region 17,18 , which is consistent with the observation that CR3022 does not block RBD binding to ACE2. Another study on CR3022 has reported highly effective SARS-CoV-2 neutralizing activity that appears to arise from destabilization of the spike trimer, a novel mechanism for neutralizing SARS-CoV-2 18 . Destabilization of viral proteins by antibodies has been observed for influenza 19 and human immunodeficiency virus 20 .Mammalian, including human, antibodies generally have two chains (heavy and light), but camelids, in addition to two-chain antibodies, also possess a single-heavy-chain antibody variant 21 .

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