Enhanced fitness of SARS-CoV-2 variant of concern B.1.1.7, but not B.1.351, in animal models. Enhanced fitness of SARS-CoV-2 variants of concern

Emerging variants of concern (VOCs) currently drive the SARS-CoV-2 pandemic. Phenotypic characterization requires advanced cell culture and animal models. We assessed VOC B.1.1.7, which rapidly became prevalent in several countries, and VOC B.1.351, which showed immune escape. In human airway epithelial cultures, replication of B.1.1.7 was not enhanced whereas B.1.351 showed reduced replication. In a competitive transmission experiment in hamsters, B.1.351 showed reduced fitness versus B.1.1.7 and the ancestral variant (wt-S614G), while fitness of B.1.1.7 and wt-S614G was similar. However, we observed a clear fitness advantage of B.1.1.7 in ferrets and two humanized ACE2 mouse models that is largely, but not completely determined by the spike protein. Our data indicate adaptation of B.1.1.7 towards increased replication in the upper respiratory tract, enhanced transmission, but not to enhanced pathology.