San Francisco Chronicle - Friday, April 27, 2007
Sabin Russell, Chronicle Medical Writer

Serosorting is choosing to have unprotected anal intercourse only with partners of the same HIV status -- uninfected men having sex only with HIV-negatives, while infected men seek out only HIV-positive partners.

The practice evolved in the gay community without the kind of institutional support given to programs encouraging condom use and reducing the number of sexual partners.

In research published today in the American Journal of Public Health, Dennis Osmond and colleagues at UCSF found that 27 percent of gay men in San Francisco practiced serosorting in 2002, compared to 19 percent in 1997. Forty percent of men ages 18 to 29 reported serosorting in 2002.

"There seems to be a tendency favoring serosorting over condom use, especially among younger men," said Osmond.

Just how protective against HIV transmission the practice may be is unknown. It clearly reduces risk as long as both partners know their infection status and disclose it. But because of the uncertainty, the practice is considered a sexual behavior that potentially puts the uninfected at higher risk.

Higher-risk sexual behaviors have increased in the gay community, Osmond found, with the highest risk concentrated among men ages 30 to 50. Despite the trend toward serosorting, the percentage of men reporting unprotected anal intercourse with a partner of different or unknown HIV status rose to nearly 15 percent in 2002 from 9 percent in 1997.

Osmond's survey shows that 27 percent of San Francisco's gay men were HIV- positive in 2002, a 40 percent increase from the 20 percent infection rate found five years earlier. The huge increase is not surprising, however, because the period covers the time when new drugs dramatically reduced the death rate, so more men were living with the virus.

A second survey led by Dr. Sandra Schwarcz of the San Francisco Department of Public Health also found that about one-quarter of the city's gay men are HIV- positive. Both studies were conducted by telephone survey involving thousands of men randomly selected, and the infection rates were comparable to those found in similar studies by the federal Centers for Disease Control and Prevention.

Schwarcz's study also detected a new trend in sexual behavior among the city's gay men. It found that those with "primary partners" -- couples who shared an emotional commitment to each other "above all others" -- were less likely to share risky sexual behaviors with each other than partners in more casual relationships.

Dr. Mitch Katz, director of the San Francisco Department of Public Health and a co-author of that study, said this is a significant change in behavior from the early days of the epidemic, when the risk of transmission was highest among men who had sex in committed relationships.

"The higher risks occurred with the more intimate relationships," he said. "People took their highest risk with their closest partner."

With untreatable AIDS rampant in the community, gay men tended to behave more cautiously in sexual encounters with strangers, while abandoning safer sex with those they held dearest. The latest study suggests a reversal of that trend, and although such behavior could lead to a higher rate of new infections, a CDC surveillance study last year showed the rate of infection is going down.

A quarter of the gay population is infected with HIV, but a high percentage of those infected are taking antiviral drugs that are believed to reduce the ability to transmit the virus. That reduction in infectiousness, combined with serosorting, may have reduced the infection rate.
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