Newly acquired HIV infections are key to transmission
Graham McKerrow, HIV i-Base
The rate of HIV transmissions in heterosexual sex was highest during early-stage infection, in a retrospective study of 235 monogamous, HIV discordant couples in a Ugandan population-based cohort.  Observers are already pointing to the study as evidence for the need for new approaches to HIV prevention strategies and the authors themselves conclude that their findings have implications for HIV prevention and for projecting the effects of antiretroviral treatment on HIV transmission.
The US-Ugandan study enrolled 15,127 adults into a community randomised trial of STD control for AIDS Prevention in the Rakai district of Uganda. Wawer and colleagues retrospectively identified 235 monogamous HIV discordant couples and estimated rates of transmission per coital act by the index partner’s stage of infection – recent seroconversion, prevalent or late-stage infection – and the adjusted rate ratio of transmission per coital act was estimated by multivariate Poisson regression.
After serocinversion of the index partner, the rate of transmission (0.0082 per coital act) within the first 2.5 months was almost 12-fold higher than that observed in the prevalent index couples (0.0007 per coital act). The rate increased significantly again at about two years before the index partner’s death.
The overall rate of transmission observed in these couples is consistent with previous estimates from Rakai, Europe and North America, but this analysis provides the first empirical data on the substantial variation in transmission by stage of infection.
The data were collected from stable, heterosexual couples, whose primary risk was through vaginal intercourse, and additional studies are required to examine transmission by stage of infection in other epidemic settings. “Nonetheless, our data have a number of clinical and epidemiological implications,” write the authors
The highest rate of transmission per coital act and the highest proportion of transmissions occurred at a time when few seroconverters know their HIV status or receive antiretroviral treatment. “Thus,” write Wawer and colleagues, “ART, initiated relatively late during infection, under current guidelines, may have only a modest impact on HIV transmission. Also, because most HIV transmissions occur before index cases are eligible to receive ART, the heterosexuals spread of drug-resistant HIV may be modest in this population. Measures that prevent primary HIV infection or reduce early viraemia (as may occur with HIV vaccines) are likely to have a greater effect than ART on the spread of HIV.”
They also draw attention to the advantages of increasing efforts to identify people with early-stage infection in order to promote safer behaviour and to consider the provision of treatment.
An editorial commentary in the Journal of Infectious Diseases says: “Wawer et al have confirmed the remarkable threat of HIV transmission posed by people with newly acquired HIV infection. The challenge now is to waste no time in finding the most creative strategies to incorporate these results into global HIV prevention efforts.” 
Both the editorial commentary and the full article are available on-line as free articles.
The point of including this article was not to suggest that antiretrovirals should be used to slow down transmission on an epidemiologically relevant level. Condoms are still the best and most and cost effective approach with any stage of HIV-infection; the primary focus of antiretrovirals is always to treat HIV-positive patients for their own healthcare.
It does however provide important data to support what forward thinking clinicians and prevention workers have realised for a long time: that a key driving force behind the epidemic is the behaviour of recently infected, undiagnosed individuals, who consider themselves HIV-negative, and behave as such, while in fact being the most highly infectious group in the population.
Recent behavioural studies – including a study presented at the BHIVA meeting in Dublin – show that receiving an HIV diagnosis leads to more extensive behaviour change towards protecting partners of unknown status than any other intervention. 
- Wawer M, Gray R, Sewankambo N et al. Rates of HIV-1 Transmission per Coital Act, by Stage of HIV-1 Infection, in Rakai, Uganda. The Journal of Infectious Diseases 2005;191:1403-1409
- Cohen M and Pilcher C. Editorial Commentary: Amplified HIV Transmission and New Approaches to HIV Prevention. The Journal of Infectious Diseases 2005;191:1391-1393
- Fox J, McClure M, Weber J et al. Risk factors for the acquisition of HIV in individuals known to have recently seroconverted. 11th BHIVA Conference, 20-23 April 2005, Dublin. Oral abstract O15.