The heterosexual infectivity of HIV is often cited as a fixed value of 1 transmission per 1000 sexual contacts, but most studies estimating this value were conducted among stable couples with a low prevalence of risk factors. These risk factors can multiply the chances of transmission by a factor of between several times and several hundred times. These are the conclusions of authors of a study published early Online and in the September issue of The Lancet Infectious Diseases. The findings are being announced at the International AIDS Conference in Mexico City.
Kimberly Powers, University of North Carolina, USA, and colleagues reviewed published data on HIV (up to April 2008) in order to estimate the effects of so-called 'transmission cofactors', ie, factors which can affect risk of heterosexual HIV transmission. Their results were extremely varied, with estimates ranging from zero transmissions after more than 100 penile-vaginal contacts in some serodiscordant* couples, to one transmission for every 3.1 episodes (or 322 infections per 1000) of heterosexual anal intercourse — more than 300 times the fixed heterosexual infectivity rate commonly quoted. Infectivity differences, expressed as number of transmissions per 1000 contacts, were 8.1 comparing uncircumcised to circumcised susceptible men (13.2 vs 5.1 infections per 1000 contacts), 6.0 comparing susceptible individuals with and without genital ulcer disease (7.5 vs 1.5), 1.9 comparing late-stage** to mid-stage index cases (2.6 vs 0.7), and 2.5 comparing early-stage to mid-stage index cases (3.2 vs 0.7).
The authors say: "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 0.001 (ie, 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 0.1 (one transmission per ten contacts) for penile-vaginal contact or even 0.3 (one transmission per 3 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 other context where the possibility of HIV exposure exists."
They conclude: "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."
"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."***
Kimberly Powers, University of North Carolina, USA T) +1 505 660 4318 E) powersk@email.unc.edu
Notes to editors: *Serodiscordant couples are those in which one partner is HIV-positive and the other HIV-negative.
** The concentration of virus in an infected person's blood and genital secretions typically varies over time according to the following pattern: the viral concentration is sharply elevated during the first few months of infection, and then falls to a lower level for several years before rising again to higher levels when the person is in later stages of infection. The probability that an HIV-infected person will transmit the infection via sexual contact is associated with viral concentrations – especially in genital secretions – so the probability that an HIV-infected person (or "index case") will transmit the infection via sexual contact is higher early and late in infection (when "viral load" is high) as compared to the middle period (when viral load is relatively lower).
***direct quote from the author – cannot be found in text of paper
for full study contact:
UK and rest of world: tony.kirby@lancet.com
US and Canada, Martine Persico: M.Persico@elsevier.com
Journal
The Lancet Infectious Diseases