Although individuals with chronic HIV infection harbor a swarm of viral quasispecies, heterosexual transmission appears to occur predominantly through acquisition of a single genetic variant. It has not been clear whether such founder viruses are random genetic variants or there is a selection bias for fitter ones. To address this question, researchers conducted a study involving participants in the Zambia Emory HIV Research Project discordant couples cohort who had intracouple HIV transmission after enrollment.
Plasma samples were obtained from 137 chronically infected donors and their newly infected partners shortly after the estimated transmission date. Comparison of viral sequences between the donors and their partners revealed a clear bias for transmission of high-fitness viruses matching the consensus of the study population. Such selection bias was decreased in situations involving increased transmission risk — for example, exposure to a donor with a high viral load (in male-to-female transmission) and, in men, the presence of genital ulcers or inflammation (in female-to-male transmission).