No current evidence for risk of vaccine-driven virulence evolution in SARS-CoV-2

Vaccines that reduce clinical severity but not infection or transmission could drive the evolution of increased rates of pathogen-inflicted damage, or virulence. Preliminary evidence suggests that COVID-19 vaccines might have such differential effects, conferring greater protection in the lower respiratory tract, where viral growth most impacts severity, than in the upper respiratory tract, where infection is chiefly determined. However, the evolution of increased virulence can only occur under certain conditions, which include the existence of a positive association between transmission and severity linked to viral genetic variation. Here, we review the current evidence for these conditions, which does not point to a risk of vaccine driven virulence evolution. An evo-epidemiological model also indicates that upper respiratory tract protection can minimize or negate selection for increased virulence should these conditions be met. Despite low apparent risks, SARS-CoV-2 virulence should be monitored, and transmission-limiting characteristics should be prioritized for second-wave vaccines.