Unravelling the origin of SARS-CoV-2: is the model good?

MacLean and colleagues recently published a very elegant analysis demonstrating that SARS-CoV-2 carries signs of positive selection and that it was already adapted to humans prior to the emergence of COVID-19. Using the Spillover theory as a reference model for zoonotic emergence, they conclude that SARS-CoV-2 must have acquired this human adaptation in bats. We reinterpreted the data from MacLean et al. using a different model of zoonotic emergence as reference, the Circulation model. The use of the Circulation model provides a more parsimonious interpretation showing that this adaptation to humans occurs in the human population after primo infection.