Health officials in San Francisco are "scrambling" to explain an apparent decline in annual HIV incidence among the city's men who have sex with men, the New York Times reports (Murphy, New York Times, 8/18). A CDC study released in June found San Francisco's HIV incidence rate among MSM has nearly halved in the last four years. The study, based on a survey of 365 MSM who were tested in the city, found an annual incidence rate of 1.2%, compared with city epidemiologists' previous estimate of 2.2%. The study led the San Francisco Department of Public Health's Office of AIDS to analyze data sets collected by the Stop AIDS Project and surveys of new cases at city clinics, both of which indicated a similar decrease in the number of new HIV cases (Kaiser Daily HIV/AIDS Report, 7/21). Willi McFarland, director of the HIV seroepidemiology unit at the San Francisco Department of Public Health, said, "We interpret this CDC report as good news, and there are some other encouraging trends and results of other studies," adding, "But we definitely do need to corroborate and independently confirm any decrease in HIV incidence by carefully examining other data. We also need to figure out if this is true, then why."

Possible Reasons
Some health officials attribute the decline in incidence to conventional efforts, such as HIV treatment programs, more frequent HIV testing, educational meetings and workshops and harm reduction strategies that work to reduce crystal methamphetamine usage, which is blamed for helping spread the virus. But other experts say that an increase in the number of MSM who know their HIV-positive status and who search for HIV-positive partners on matchmaking Web sites designed specifically for them might be contributing to the decrease in new HIV cases. The dating practice, which is called "sero-sorting," involves men choosing sex partners based on their common HIV serostatus, which refers to the presence of antibodies to a particular infectious agent in the blood, according to the Times. "Studies have shown when people have knowledge of their serostatus, they take that knowledge and use it to protect their partners," Patrick Sullivan, chief of CDC's behavioral and clinical surveillance branch, said, adding, "Sero-sorting is one piece of that whole benefit that arises from people learning their status through HIV testing." Sullivan called the CDC report a "snapshot in time" and noted that directly comparable data will not be available until another survey is conducted in 2007 (Murphy, New York Times, 8/18).

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