A new study has revealed that the level of HIV-1 in the blood of an HIV-infected partner is the single most important factor influencing risk of sexual transmission to an uninfected partner.
James P. Hughes, PhD, and colleagues at the University of Washington and the Fred Hutchinson Cancer Research Center, in Seattle; the University of Witwatersrand in South Africa; the University of Nairobi and Kenyatta National Hospital, in Kenya; and the Rwanda-Zambia HIV Research Group were behind the study.
The study included 3,297 HIV-discordant couples (where one person is HIV-infected, and the other is not) in eastern and southern Africa who were enrolled in a randomized trial of acyclovir suppressive therapy.
The couples had frequent follow-up to measure plasma HIV-1 RNA in the infected partner and genetic testing to link the transmitted virus to the index HIV-infected partner, to prevent inclusion of infections acquired from other possible partners.
HIV acquisition was not affected by the acyclovir therapy.
The study confirmed that condoms are highly protective, reducing the risk of HIV transmission by 78 percent when subjects reported using a condom.
The authors said what was most important was the level of HIV-1 RNA in the blood of the infected partner.
The higher the viral load in the index infected partner, the higher the risk of transmission, emphasizing the importance of lowering viral load to help prevent the spread of HIV-1 through sex.
Older age was associated with reduced transmission per sex act, and male circumcision reduced female-to-male transmission by approximately 47 percent.
Genital herpes infections and the presence of genital ulcers were associated with increased rates of transmission.
"Our results underscore the importance of antiretroviral therapy, and, possibly, treatment of co-infections, to reduce plasma HIV-1 viral load in HIV-1 infected partners, and condom promotion, male circumcision, and treatment of symptomatic sexually-transmitted infections for HIV-1 uninfected partners as potential interventions to reduce HIV-1 transmission," the authors wrote.
The findings also showed that the risk of an HIV-infected man transmitting an infection to a woman not infected with HIV was about twice the risk of an HIV-infected woman transmitting to an HIV-uninfected man.
However, this difference can be attributed to the difference in viral loads between men and women, the authors noted.
On average, HIV-infected men have higher HIV-1 loads.
Difference in age and having genital herpes in the HIV-uninfected partners also help account for the disparity-the HIV-uninfected female partners were, on average, younger and had higher rates of genital herpes than their male counterparts.
The study is published in the Journal of Infectious Diseases.