Enhanced fitness of SARS-CoV-2 B.1.617.2 Delta variant in ferrets

Competition assays were conducted in vitro and in vivo to examine how the Delta (B.1.617.2) variant displaced the prototype Washington/1/2020 (WA/1) strain. While WA/1 virus exhibited a moderately increased proportion compared to that in the inoculum following co-infection in human respiratory cells, Delta variant possessed a substantial in vivo fitness advantage as this virus becoming predominant in both inoculated and contact animals. This work identifies critical traits of the Delta variant that likely played a role in it becoming a dominant variant and highlights the necessities of employing multiple model systems to assess the fitness of newly emerged SARS-CoV-2 variants.