(RxWiki News) We all know that HIV is a sexually transmitted disease, and that safe sex is the best way to prevent infection. But if you're not being safe with an HIV-positive partner, what's your risk?
A new study has found that the level of HIV-1 in the blood of the infected partner is the most important factor influencing their partner's risk of getting the virus.
The study estimates that infection occurs once per every 900 sex acts, in couples where one partner is HIV-positive and the other is not.
The study reconfirmed that condoms provide a high level of protection against the virus, reducing transmission by 78 percent.
"If your partner is HIV positive, use a condom."
The study was conducted by Dr. James Hughes of University of Washington, and carried out in collaboration with several research groups in eastern and southern Africa.
Previous studies had attempted to estimate the “infectivity” of HIV, or the probability of transmission per coital act. This information is very important for scientists trying to estimate how the AIDS epidemic will spread or decline, and understanding how effective intervention efforts are for preventing infections.
But it proved difficult for studies to come up with consistent numbers. Dr. Hughes' study approached the problem by studying a huge number of couples – approximately 3,300 couples participated.
Each couple had one partner who was HIV-positive and one who was not. The research team got quarterly updates from the couple, and also used genetic analysis to make sure that if the HIV-negative partner became infected, the virus came from the main partner.
The study found that viral load is the main driver of transmission. Viral load reflects the severity of the infection, and can increase and decrease depending on treatment.
The higher the viral load, the higher the risk that the partner would be infected.
"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.
That risk was decreased by 78 percent when the couple used condoms. Overall, the study authors estimated that infection occurs once per 900 coital acts.
Another interesting finding was that the risk of an HIV-infected man passing the virus to an uninfected woman was two times higher than the virus going from a woman to a man. Commonly, HIV-infected men have higher viral loads than women.
Although viral load is the main influence for transmission risk, there are many other factors contributing to infection including gender and other sexually transmitted diseases like genital herpes.
There are some caveats to the study, discussed in an editorial commentary that accompanied the paper in the Journal of Infectious Disease.
Drs. Ronald Gray and Maria Wawer say the estimate of infectivity is too low to explain the explosive epidemic seen in Sub-Saharan Africa, and that infectivity could be much higher during early or late stage infections, which were not measured during the study.
The study was published in January 2012.