Emerging variants of concern (VOCs) are driving the COVID-19 pandemic^1,2. Experimental assessments of replication and transmission of major VOCs and progenitors are needed to understand the mechanisms of replication and transmission of VOCs³. Here we show that the spike protein (S) from Alpha (also known as B.1.1.7) and Beta (B.1.351) VOCs had a greater affinity towards the human angiotensin-converting enzyme 2 (ACE2) receptor than that of the progenitor variant S(D614G) in vitro. Progenitor variant virus expressing S(D614G) (wt-S^614G) and the Alpha variant showed similar replication kinetics in human nasal airway epithelial cultures, whereas the Beta variant was outcompeted by both. In vivo, competition experiments showed a clear fitness advantage of Alpha over wt-S^614Gin ferrets and two mouse models-the substitutions in S were major drivers of the fitness advantage. In hamsters, which support high viral replication levels, Alpha and wt-S^614Gshowed similar fitness. By contrast, Beta was outcompeted by Alpha and wt-S^614Gin hamsters and in mice expressing human ACE2. Our study highlights the importance of using multiple models to characterize fitness of VOCs and demonstrates that Alpha is adapted for replication in the upper respiratory tract and shows enhanced transmission in vivo in restrictive models, whereas Beta does not overcome Alpha or wt-S^614Gin naive animals.

Data Availability Statement: Sequence data are available on the NCBI Sequence Read Archive (SRA) under the accession numbers PRJEB45736 and PRJNA784099, or in GenBank under the accession numbers MT108784, MZ433432, OL675863, OL689430 and OL689583 as shown in Extended Data Table ?Table11. Source data are provided with this paper.