New HIV infections in San Francisco may be half of official estimates

The rate of new HIV infections resulting from sex between men in San Francisco may need to be revised to about half of that of offical estimates, according to new data from the United States Centres for Disease Control (CDC), San Francisco's sexual health clinics and an anonymous HIV testing centre in the city where official estimates of HIV incidence have remained at 2.2% since 2000, after peaking at 3.7% 1999 from a low of 1.3% in 1997.

The impetus for the revised figures was a study from the CDC published last month in MMRW Weekly which analysed data from five of 17 cities participating in the National HIV Behavioural Surveillance system.

Overall, of the 1767 gay and bisexual men surveyed, 25% were found to be infected with HIV (after giving voluntary blood samples), and 48% of those infected were unaware of their infection. Unrecognised HIV infection was highest in men younger than 30 and of non-white ethnicity. But only 23% of the men surveyed in San Francisco were unaware of their infection, compared with 62% in Baltimore and 52% in New York City.

HIV incidence in San Francisco was calculated as 1.2% (95% CI, 0.0%-2.6%), the lowest of the five cities included in the analysis. San Francisco epidemiologists had previously estimated an incidence rate of 2.2%, or more than 1,000 new HIV infections annually.

In contrast, Baltimore's HIV incidence rate was 8.0% (95% CI, 4.2%-11.8%); Miami's 2.6% (95% CI, 0.0%-5.6%); New York City's 2.3% (95% CI, 0.28%-4.2%); and Los Angeles' 1.4% (95% CI, 0.0%-2.9).

However, HIV prevalence (the total percentage of those infected) in San Francisco was, at 24%, the second-highest, after Baltimore's 40%.

It should be noted that the CDC's findings are preliminary and were not weighted by venue-selection probability (all of the men in the survey were recruited in gay-identified locations like bars, clubs and gay neighbourhoods). Also, the relatively small number of men included in the survey (365 in San Francisco and between 222 and 462 in other cities) cannot provide firm epidemiological data.

Nevertheless, the data were intriguing enough for the Department of Public Health's Office of AIDS to request other locally-acquired surveillance data. According to an article in yesterday's San Francisco Chronicle HIV incidence rates pooled from San Francisco's sexual health clinics fell to 3.2% from a high of 5.4% in 2000. Similarly, HIV incidence rates at a San Francisco HIV anonymous testing centre declined from 3.9% to 2.8% over the same period.

"It's wonderful news, but we can't explain why," Eileen Shields from San Francisco's Department of Public Health was reported as saying on the website 365Gay.com.

San Francisco's health director, Dr Mitch Katz, is reported to have suggested to the San Francisco Chronicle that "the most likely explanation is that effective AIDS drugs have lowered the level of virus in those men who are HIV-positive and still having unprotected sex."

Jason Riggs from San Francisco's Stop AIDS Project told several media outlets that the Project's surveys have shown that less HIV-positive men (21% vs. 31% in 2001) and HIV-negative men (4% vs. 20% in 2001) are having unprotected sex with partners of different or unknown status.

Riggs also suggests that public health campaigns aimed at curbing methamphetamine use are now paying off. "We know that crystal meth accounts for about 30% of new HIV infections," he told a local TV station, "and so the treatment and prevention programs for crystal meth abuse could be having a very dramatic impact on this decrease or possible decrease."

However, Eileen Sheilds told 365Gay.com that HIV incidence rates seen in the CDC study may be due to "differences in reporting methods, the size of the sample the CDC used or any number of other factors."

Next month, a panel of epidemiology experts will meet to decide whether to officially revise the annual figures. They will use at least eleven different indicators to provide an official consenus on HIV incidence. Nevertheless, even if the official figures are lowered, Dr. William McFarland, an epidemiologist working at San Francisco's Department of Public Health's Office of AIDS told the San Francisco Chronicle that "the incidence rate is still too high. There is a lot more work to be done."