Watching SARS-CoV-2 evolve is fascinating and frightening

GENOME SEQUENCING, crucial to the original identification of SARS-CoV-2 and to providing the templates for the vaccines against it, also offered a new way to watch the pandemic unfold. Viruses often make mistakes when they copy their genomes, a crucial step in reproduction, and that means they accumulate mutations pretty quickly. Sequence enough samples over time and you can see evolution in action. For the first six months of the pandemic this was a matter of some academic interest—and more than a little foreboding. Last autumn the bad news that students of evolution had been fearing began to appear in the data.