The TWiVotopes review current evidence which does not show the need for widespread COVID-19 booster vaccination, and results of experiments to examine the replication and immune evasion of the SARS-CoV-2 delta variant.

Shane Crotty returns to TWiV to review the immunology of COVID-19, including differences between infection and vaccination, increased breadth of antibodies after infection followed by vaccination, the roles of T cells, and whether booster vaccinations are needed.

The meteorologicomaniacs discuss the COVID-19 pandemic global death toll, effectiveness of vaccines against the delta variant, and how analysis of ancient viral DNA reveals that two pathogenic human viruses were brought into Mexico during the transatlantic slave trade.

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.