TUESDAY, Aug. 5 (HealthDay News) -- A widely used HIV infectivity rate doesn't take into account multiple risk factors, say U.S. researchers who reviewed published data.

The heterosexual infectivity of HIV (the virus that causes AIDS) is often cited as a fixed value of one transmission per 1,000 sexual contacts. However, most studies estimating this value were conducted among stable couples with a low prevalence of risk factors, which can increase the risk of HIV transmission by several to several hundred times, according to Kimberly Powers, of the University of North Carolina, and colleagues.

They reviewed published data on HIV (up to April 2008) in order to estimate the effects of transmission co-factors that can affect the risk of heterosexual HIV transmission.

They found wide variations, with estimates ranging from zero transmissions after more than 100 penile-vaginal contacts in some serodiscordant couples (one partner HIV-positive and one partner HIV-negative), to one transmission for every 3.1 episodes of heterosexual anal intercourse, which is more than 300 times the commonly-cited fixed heterosexual infectivity rate.

Other infectivity differences, expressed as number of transmissions per 1,000 contacts, were:

• 13.2 for uncircumcised susceptible males vs. 5.1 for circumcised males.
• 7.5 for susceptible people with genital ulcer disease vs. 1.5 for those without such disease.
• 3.2 for early-stage index cases, 2.6 for late stage index cases, and 0.7 for mid-stage index cases.

"The use of a single, 'one-size-fits-all' value for the heterosexual infectivity of HIV-1 obscures important differences associated with transmission cofactors. Perhaps more importantly, the particular value of 0001 (i.e., one infection per 1000 contacts between infected and uninfected individuals) that is commonly used seems to represent a lower bound. As such, this value substantially underestimates the infectivity of HIV-1 in many heterosexual contexts...heterosexual infectivity can exceed 01 (one transmission per 10 contacts) for penile-vaginal contact or even 03 (one transmission per three contacts) for penile-anal contact. Claims in both the popular media and the peer-reviewed literature that HIV is very difficult to transmit heterosexually are dangerous in any context where the possibility of HIV exposure exists," the review authors wrote.

"Improved infectivity estimates -- especially more detailed estimates that quantify the amplifying effects of biological cofactors -- will help us to grasp the magnitude of the HIV epidemic, accurately communicate the level of risk involved in heterosexual sex, and identify the best possible intervention strategies for slowing the epidemic's spread," they concluded.

"Infectivity studies are very difficult to conduct, and that few studies exist as a result. Many of the studies producing the published estimates suffered from at least one potential bias. Therefore, while our study documents the considerable heterogeneity of the heterosexual infectivity of HIV-1 and provides some explanations for this heterogeneity, considerable uncertainty remains."?

The findings were expected to be announced at the International AIDS Conference in Mexico City. The review was published online in The Lancet Infectious Diseases and was expected to be in the September print issue of the journal.

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