TWiV discusses whether or not Omicron should be designated a new serotype of SARS-CoV-2, and the finding that amino acid changes that adapt SARS-CoV-2 to mink or ferret do not increase fitness in the human airway.

TWiV reveals all that we know so far about the variant of concern B.1.1.529, also called Omicron, and describes a study on the ability of the alpha, beta, and delta variants to reproduce in cells in culture and induce membrane fusion.

TWiV reviews a leaked CDC document on virulence and spread of the delta variant, and how anti-alphavirus antibodies that do not neutralize infection can nevertheless protect mice from disease.

Ron Fouchier explains why increased transmission of SARS-CoV-2-variants has not been demonstrated, and their ability to displace ancestral variants is due to greater fitness, much like influenza virus antigenic variation leads to better fitness and replacement of previously circulating variants.