The SARS-CoV-2 B.1.617 lineage was identified in October 2020 in India1–5. It has since then become dominant in some indian regions and UK and further spread to many countries6.

The lineage includes three main subtypes (B1.617.1, B.1.617.2 and B.1.617.3), harbouring diverse Spike mutations in the N-terminal domain (NTD) and the receptor binding domain (RBD) which may increase their immune evasion potential.

B.1.617.2, also termed variant Delta, is believed to spread faster than other variants. Here, we isolated an infectious Delta strain from a traveller returning from India. We examined its sensitivity to monoclonal antibodies (mAbs) and to antibodies present in sera from COVID-19 convalescent individuals or vaccine recipients, in comparison to other viral strains.

Variant Delta was resistant to neutralization by some anti-NTD and anti-RBD mAbs including Bamlanivimab, which were impaired in binding to the Spike.

Sera from convalescent patients collected up to 12 months post symptoms were 4 fold less potent against variant Delta, relative to variant Alpha (B.1.1.7).

Sera from individuals having received one dose of Pfizer or AstraZeneca vaccines barely inhibited variant Delta.

Administration of two doses generated a neutralizing response in 95% of individuals, with titers 3 to 5 fold lower against Delta than Alpha. Thus, variant Delta spread is associated with an escape to antibodies targeting non-RBD and RBD Spike epitopes.

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