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Can Resistant Viruses Be Transmitted?

August 8, 2000

As drug resistance becomes more of a factor in deciding which anti-HIV medications to use, an important question becomes, "How common are pre-existing mutations that cause resistance to drugs that people have never taken?" There have been compelling studies in the past to show that people can become infected with a virus that is already resistant to anti-HIV drugs. The question being addressed now is, "How common is this?" Following are summaries of data from several studies, all investigating the rate of drug-resistant virus in people who are HIV-positive, but have never taken any anti-HIV medications.

Data collected by the U.S. Centers for Disease Control and Prevention from 10 U.S. cities from 1997 to 1999 were presented by Weintock and colleagues at the conference in Durban. All of the people in this study were treatment-naive (had never been on anti-HIV drugs) and were recently diagnosed with HIV. The study found that of the 209 people included in the study, 4% had mutations that could cause resistance to nucleoside reverse transcriptase inhibitors (NRTIs), and only 1% had mutations to non-nucleoside reverse transcriptase inhibitors (NNRTIs) and protease inhibitors (PIs). The study concluded that overall only 5% of the people studied had some type of mutation that could lead to resistance, and that there was no increase in the prevalence of pre-existing resistance over the three-year period of the study.

This information was slightly different from that presented by Grant and colleagues on a study conducted in San Francisco from 1996 to 1999. In this study, which included 118 treatment-naive HIV-positive individuals, there was a decrease over the time of the study in the number of people with pre-existing mutations for NRTI resistance, from 15% to 6.3%, while there was an increase in the number of people with mutations for PIs and NNRTIs, from 0% to 6.3%. This probably reflects the increased use of both PIs and NNRTIs in the San Francisco area following their approval in 1996.

In a study conducted in Seattle and Los Angeles, pregnant women who were newly infected with HIV and treatment-naive were tested for resistant mutations. About 10% of the women in this study had mutations that could cause resistance to NRTIs, NNRTIs or PIs, but all of the women with virus that contained these mutations were from the Seattle area, not Los Angeles.

The prevalence of mutations in untreated people living with HIV varied greatly in each of the studies discussed above, and can seem quite contradictory. These differences suggest that viruses with pre-existing mutations may be more common in particular geographic areas. This difference could be due to any number of factors, one of which could be the number of people in an area with a history of early, extensive, or sub-optimal therapy experience. Additionally, some people spontaneously develop minor mutations to the NNRTIs which do not appear to actually cause resistance once drugs are used. Thus, some studies may overestimate the incidence of transmission of drug-resistant HIV if they include these minor NNRTI mutations.

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