Fig. 2. Delta variant live virus replication kinetics and spike-mediated infectivity.

a–d, Live virus replication comparing B.1.1.7 with B.1.617.2. Calu-3 cells were infected with variants at an MOI of 0.1. a, Viral loads measured by qPCR in cell lysates. b, Viral protein levels in cell lysates. c, d, Live virus produced from infected Calu-3 cell supernatants was collected and used to infect permissive Vero E6 ACE2/TMPRSS2 cells to measure viral loads (c) or TCID₅₀ ml⁻¹ (d). e, f, Virus replication kinetics in the HAE system. g, Live virus replication in airway epithelial organoid cultures. Airway epithelial organoids were infected with the SARS-CoV-2 variants B.1.1.7 and B.1.617.2 at an MOI of 1. Cells were lysed and total RNA was isolated. qPCR was used to determine the number of copies of the nucleoprotein gene in cells and the infectivity of cell-free virus measured by infection of Vero E6 ACE2/TMPRSS2 cells. The data are representative of two independent experiments. dpi, days post-infection. h, i, Western blots of PV virions (h) and cell lysates (i) of 293T producer cells following transfection with plasmids expressing lentiviral vectors and SARS-CoV-2 S B.1.617.1 and Delta variant B.1.617.2 versus WT (Wuhan-1 with D614G), probed with antibodies to HIV-1 p24 and SARS-Cov-2 S2. j, Calu-3 cell entry by spike B.1.617.2 and B.1.617.1 versus WT D614G parental plasmid PVs. The data are representative of three independent experiments. k, Growth kinetics of B.1.617.1 and B.1.617.2 variants. Viral isolates of B.1.617.1 and B.1.617.2 were inoculated into Calu-3 cells, and viral RNA in the culture supernatant was quantified by real-time RT–PCR. The TCID₅₀ of released virus in supernatant was measured over time. Assays were performed in quadruplicate. NS, not significant; *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. The data are representative of two independent experiments. Uninfected cells are represented by a minus symbol. NP, nucleocapsid protein.