Researchers with the Public Library of Science said on Thursday that genetic mutations in HIV reduce the genetic diversity of the virus when it infects a patient, and patients may respond differently.
Pleuni Pennings, lead researcher, said that in some patients, a drug-resistance mutation appeared in a single virus particle, which then rapidly moved to the entire viral population within the patient. In other patients, the same resistance mutation occurred in multiple viral particles, which led to a more heterogeneous, and still drug-resistant, viral population.
HIV researchers are concerned with the question of the "effective population size" of a virus. It is a mathematical number that determines how quickly drug resistance may evolve.
Pennings and researchers observed that drug resistance comes about by hard and soft selective sweeps.
In a hard sweep, the entire resistant population is made of a single virus particle's descendants. In a soft sweep, it consists of progeny of different virus particles. The two sweeps have distinct impacts on the population.
In the current study, Pennings re-analyzed a data set from the 1990s, focusing on a subset of patients where the development of resistance could be observed. He determined that both hard and soft sweeps of the virus occur in HIV.
For future studies, Pennings plans to study patients who were treated with other drugs to understand which drugs are more effective in preventing resistance.