ACE2 and infection: In order to examine whether the increased affinity of the N501Y variant for ACE-2 was associated with a more efficient establishment of infection and development of disease, we challenged transgenic ACE-2 humanized K18-hACE2 mice with the early 2020 SARS-CoV2 B.1 (Freiburg isolate, FR-4248 Miladi, 2020) and the B1.1.7 (alpha) strains.