Common estimate of heterosexual HIV transmission risk sometimes far too low
Mark Mascolini for NATAP.org
…In heterosexual couples with enough other risk factors, transmission risk can climb as high as 1-in-10 for penile-vaginal sex and 1-in-3 for penile-anal sex…..heterosexual sex can be a remarkably efficient way to transmit HIV….
One commonly cited estimate of heterosexual HIV-1 transmission risk – 1 infection per 1000 sexual acts – is probably inaccurate because it fails to account for other factors that raise or lower the risk of HIV transmission. 
That conclusion emerged from a multi-study analysis by Kimberly Powers (University of North Carolina at Chapel Hill), who pinpointed five variables that have a potent impact on heterosexual HIV transmission. With enough cofactors in play, Powers estimated that men and women risk transmitting the virus once every three times they have insertive sex.
The investigators suspected the 1-per-1000 ratio may be too low to explain raging heterosexual HIV epidemics in many countries, partly because it does not factor in sexually transmitted infections (STIs), HIV disease stage, circumcision, and other variables known to boost or blunt transmission risk. Yet that ratio gets cited time and again in government reports, peer-reviewed studies, and media offerings, leaving the impression that heterosexual coitus is a highly inefficient way of infecting a partner. To get a better handle on sexual transmission dynamics, the North Carolina team systematically searched published studies estimating heterosexual infectivity of HIV-1. Then they used statistical tools to sort out infectivity differences according to risk cofactors.
Powers found 27 studies involving 15 distinct populations. Transmission estimates varied strikingly from one study to the next, depending on these cofactors. Estimates ranged from 0 transmissions after more than 100 penile-vaginal contacts to 1 transmission for every 3.1 episodes of heterosexual anal intercourse. The multistudy statistical analysis weighing the impact of cofactors identified five variables that boosted risk of HIV transmission:
- Transmission 33.8 times more likely with penile-anal sex than penile-vaginal sex.
- Transmission 8 times more likely for uncircumcised versus circumcised men.
- Transmission 6 times more likely with than without a genital ulcer disease.
- Transmission 2.5 times more likely with early versus mid-stage HIV infection.
- Transmission 1.85 times more likely with late versus mid-stage HIV infection.
Powers and colleagues concluded that the 1-in-1000 estimate adequately represents transmission risk only in stable couples with low rates of other transmission risk factors. In other words, 1-in-1000 “represents a lower bound” of a capacious risk spectrum. In heterosexual couples with enough other risk factors, transmission risk can climb as high as 1-in-10 for penile-vaginal sex and 1-in-3 for penile-anal sex.
The investigators encouraged researchers to consider such cofactors in future infectivity estimates, and they advised public health officials and clinicians to emphasise that heterosexual sex can be a remarkably efficient way to transmit HIV. The study will be published next week in Lancet Infectious Diseases.
1. Powers K, Poole C, Pettifor A, Cohen M. Rethinking the heterosexual infectivity of HIV-1: A systematic review and meta-analysis. 3rd International Workshop on HIV Transmission: Principles of Intervention. July 31-August 2, 2008, Mexico City. Abstract 14.