More Than 40% of HIV-Positive Individuals in San Francisco Will Have Drug-Resistant Viral Strains by 2005

Drug-resistant strains of HIV will infect 42% of HIV-positive individuals in San Francisco by 2005, according to a mathematical model developed and applied by researchers at the University of California-San Francisco and the University of California-Los Angeles, the San Francisco Chronicle reports. The percentage of people with drug-resistant HIV rose from 0% in 1996, when protease inhibitors were first introduced, to 28.5% in 1999 and will reach 42% by 2005, according to the model developed by UCLA biomathematician Sally Blower. Blower, who co-wrote the report in the September issue of Nature Medicine, has been using the model to track infectious diseases for nearly 10 years. Preliminary data from San Francisco General Hospital clinics that showed that 28% of patients had drug-resistant HIV in 1999 "suggest" that the model is on target (Russell, San Francisco Chronicle, 8/31). Resistant strains develop when drug therapy does not fully suppress HIV, allowing the virus to learn the drugs' mechanisms and mutate to form drug-resistant strains. Those strains then can be transmitted to other people through unprotected sex or intravenous drug use. According to Blower, however, the "vast majority of new cases come from acquired drug resistance, not sexual transmission" of resistant strains (Beasley, Reuters, 8/31). Blower's model estimated that 8% of new HIV cases in San Francisco last year were drug-resistant, a number consistent with the 9% reported by city clinics (San Francisco Chronicle, 8/31). Blower said that focusing on developing new drugs and getting them to market are the most "important" measures public health officials can take to combat the rise in drug-resistant strains (San Francisco Chronicle, 8/31).