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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has undergone progressive change, with variants conferring advantage rapidly becoming dominant lineages, e.g., B.1.617. With apparent increased transmissibility, variant B.1.617.2 has contributed to the current wave of infection ravaging the Indian subcontinent and has been designated a variant of concern in the United Kingdom. Here we study the ability of monoclonal antibodies and convalescent and vaccine sera to neutralize B.1.617.1 and B.1.617.2, complement this with structural analyses of Fab/receptor binding domain (RBD) complexes, and map the antigenic space of current variants. Neutralization of both viruses is reduced compared with ancestral Wuhan-related strains, but there is no evidence of widespread antibody escape as seen with B.1.351. However, B.1.351 and P.1 sera showed markedly more reduction in neutralization of B.1.617.2, suggesting that individuals infected previously by these variants may be more susceptible to reinfection by B.1.617.2. This observation provides important new insights for immunization policy with future variant vaccines in non-immune populations. [Display omitted] • Vaccine/convalescent sera show reduced neutralization of B.1.617.1 and B.1.617.2 • Sera from B.1.351and P.1 show markedly reduced neutralization of B.1.617.2 • B.1.351, P.1, and B.1.617.2 are antigenically divergent • Vaccines based on B.1.1.7 may protect broadly against current variants The B.1.617 lineage of SARS-CoV-2, especially the delta strain, which is B.1.617.2, has contributed to the wave of infection in the Indian subcontinent. Structural and serological analyses show some evidence of antibody escape, and individuals infected previously with the B.1.351 (beta) and P.1 (gamma) variants are likely more susceptible to reinfection by the delta strain. Vaccines based on B.1.1.7 (alpha) are likely to provide the broadest protection against current variants. [ABSTRACT FROM AUTHOR]