Figure 1.

Delta variant has an improved fitness over Alpha variant on the Calu-3 and primary human airway epithelial cultures

(A) Plaque morphologies of USA-WA1/2020, Alpha, and Delta viruses. The plaque images were taken on day 2.5 post infection of Vero E6 cells. The average diameters of the plaques are presented in the parentheses.

(B) Viral kinetics on Vero E6 cells. Recombinant Alpha and Delta variants were inoculated onto Vero E6 cells at an MOI of 0.01. After 1 h infection, the cells were washed thrice with DPBS to remove unattached viruses. The viral titers in the culture supernatant were detected at 12, 24, 36, and 48 h post infection. Red dots represent individual cell cultures (n = 6) pooled from 2 independent biological repeats. Data are mean ± SEM. Statistical analysis was performed using nonparametric two-tailed Mann-Whitney test.

(C) Replication competition between Delta and Alpha variants on Calu-3 cells. Culture medium was sampled for Sanger sequencing at 24, 36, and 48 h post infection.

(D) Replication competitions between Alpha and Delta variants on primary human airway epithelial (HAE) cells. Secreted progeny viruses were collected by incubating the apical side of the HAE culture with 300 μL DPBS at 37°C for 30 min from day 1 to 5. For both (C) and (D), equal PFUs of Alpha and Delta SARS-CoV-2s were mixed and inoculated onto Calu-3 and HAE cells at an MOI of 5. At 2 h post infection, the cells were washed thrice with DPBS to remove unattached viruses. Red dots represent individual cell cultures (n = 6) pooled from 2 independent biological repeats. The horizontal lines in each catseye represent the mean. Shaded regions represent standard error of the mean. y axes use a log₁₀ scale. Black numbers above each set of values (catseye) indicate the ratios of two viral RNA species. p values were calculated for group coefficient using linear regression model. ^∗∗p < 0.01, ^∗∗∗p < 0.001.