Variation and multilevel selection of SARS-CoV-2

The evolution of SARS-CoV-2 remains poorly understood. Theory predicts a group-structured population with selection acting principally at two levels: the pathogen individuals and the group of pathogens within a single host individual. Rapid replication of individual viruses is selected for, but if this replication debilitates the host before transmission occurs, the entire group of viruses in that host may perish. Thus, rapid transmission can favor more pathogenic strains, while slower transmission can favor less pathogenic strains. Available data suggest that SARS-CoV-2 may follow this pattern. Indeed, high population density and other circumstances that favor rapid transmission may also favor more deadly strains. Health care workers, exposed to pathogenic strains of hospitalized patients, may be at greater risk. The low case fatality rate on the Diamond Princess cruise ship may reflect the founder effect-an initial infection with a mild strain. A vaccine made with one strain may confer limited immunity to other strains. Variation among strains may lead to the rapid evolution of resistance to therapeutics. Finally, if less pathogenic strains are largely associated with mild disease, rather than treating all SARS-CoV-2 positive individuals equally, priority could be focused on testing and contact tracing the most seriously symptomatic patients.