HAART reduces HIV infectivity by over 60%, but increase in unsafe sex means stable HIV incidence

Michael Carter
Published: 16 January 2004

HAART has reduced the ability of HIV-positive gay men to transmit the virus by 60%, according to a study conducted by the University of California, San Francisco, published in the January 2nd edition of AIDS. However, this has been offset by an increase in the risk behaviour gay men meaning that the number of new infections has remained roughly stable.

Investigators analysed data from the San Francisco Young Men’s Health Study between 1994 and 1999. This study involved 534 young gay men (aged 18- 29), who initially tested HIV-negative. At four follow-up visits the young gay men were asked about the kind of sex they were having and were re-tested for HIV. Two of the follow-up visits were in the period before effective HAART was widely available (pre-1996), and the other two visits were in the HAART era.

After considering factors including sexual practices and condom use, the investigators estimated the risks of an HIV-negative gay man becoming infected with HIV by a sexual partner. Researchers found that the per-partnership probability of HIV transmission taking place fell from 0.120 in the pre-HAART era to 0.044 in the HAART-era, providing a 60.4% decline in HIV infectivity (p=0.028) risk per sexual partner since the use of HAART became widespread among HIV-positive gay men in San Francisco.

The investigators undertook further analysis, taking into account the increase in HIV prevalence amongst gay men in San Francisco since HAART reduced mortality. They found that not only did their initial finding remain robust, but that the decline in HIV infectivity per sexual partner fell even further to 67%.

In addition, the investigators estimated the decline in infectivity of men who had unprotected anal sex with five or more partners, assuming that the sexual partners of these individuals had a 50% higher prevalence of HIV. The decline in HIV infectivity remained statistically significant for these higher risk individuals (p=0.019), the estimated per-partnership infectivity being 0.107 in the pre-HAART era and 0.040 in the HAART era.

However, the decline in infectivity was accompanied by a doubling in the amount of unprotected anal sex, the sexual activity that carries the greatest risk of HIV transmission, reported by young gay men enrolled in the study.

The investigators conclude, "the 60% decline in HIV infectivity we observed following the introduction of HAART suggests that the use of HAART in infected persons not only confers clinical benefit, but is also an attractive tool for HIV prevention." However, the investigators emphasise that coinciding with this reduction in infectivity "we observed …an increase in unprotected sexual behaviour in San Francisco" and "community level data among gay men in San Francisco revealed a rising incidence [of HIV infections] soon thereafter. Thus, the benefit of reduced HIV transmission in the community due to widespread use of HAART may be offset by increases in unsafe sexual encounters. Use of HAART is a potentially important HIV prevention tool, one that is likely to succeed, however, only if accompanied by continued emphasis on avoidance of exposure."


Porco TC et al. Decline in HIV infectivity following the introduction of highly active antiretroviral therapy. AIDS 19: 81 – 88, 2004.

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