EXPONENTIAL INCREASES OF RNA VIRUS FITNESS DURING LARGE POPULATION TRANSMISSIONS

The great adaptability shown by RNA viruses is a consequence of their high mutation rates, Here we investigate the kinetics of virus fitness gains during repeated transfers of large virus populations in cell cu lture. Results always show that fitness increases exponentially. Low f itness clones exhibit regular increases observed as biphasic periods o f exponential evolutionary improvement, while neutral clones show mono phasic kinetics. These results are significant for RNA virus epidemiol ogy, optimal handling of attenuated live virus vaccines, and routine l aboratory procedures.