SARS-CoV-2 Spike protein is critical for virus infection via engagement of ACE21, and is a major 54 antibody target. Here we report chronic SARS-CoV-2 with reduced sensitivity to neutralising 55 antibodies in an immune suppressed individual treated with convalescent plasma, generating 56 whole genome ultradeep sequences over 23 time points spanning 101 days. Little change was 57 observed in the overall viral population structure following two courses of remdesivir over the 58 first 57 days. However, following convalescent plasma therapy we observed large, dynamic 59 virus population shifts, with the emergence of a dominant viral strain bearing D796H in S2 and 60 􀪢H69/􀪢V70 in the S1 N-terminal domain NTD of the Spike protein. As passively transferred 61 serum antibodies diminished, viruses with the escape genotype diminished in frequency, before 62 returning during a final, unsuccessful course of convalescent plasma. In vitro, the Spike escape 63 double mutant bearing 􀪢H69/􀪢V70 and D796H conferred modestly decreased sensitivity to 64 convalescent plasma, whilst maintaining infectivity similar to wild type. D796H appeared to be 65 the main contributor to decreased susceptibility but incurred an infectivity defect. The 66 􀪢H69/􀪢V70 single mutant had two-fold higher infectivity compared to wild type, possibly 67 compensating for the reduced infectivity of D796H. These data reveal strong selection on SARS68 CoV-2 during convalescent plasma therapy associated with emergence of viral variants with 69 evidence of reduced susceptibility to neutralising antibodies.