The risk of transmitting HIV is significantly reduced in people on successful antiretroviral therapy for more than six months, according to Danish investigators and reported on by aidsmap. The Danish study is published in the online edition of the journal HIV Medicine.

“In this nationwide population-based cohort study of Danish HIV-infected patients on HAART [highly active antiretroviral therapy] with more than six months of suppressed viral load, we found that the risk of experiencing a viral load above 1,000 copies/ml and thereby transmitting HIV sexually was very low,” investigators said.

Researchers found, however, that during the first year of HIV treatment, viral load was above that transmission threshold for about 5 percent of the time.

Nevertheless, the study further supports a 2008 statement made by the Swiss Federal Commission for HIV/AIDS that noted, “a seropositive person without additional sexually transmitted disease on antiretroviral treatment with suppressed viral load cannot transmit HIV sexually.” The Swiss made this recommendation based on data obtained through heterosexual couples in monogamous relationships.

Danish researchers began their study after the Swiss statement. By analyzing the viral load results of 2,680 patients reported between 2000 and 2007, they found that transmission risk was higher in the first six months of treatment. During that time, patients' viral load was above 1,000 copies/ml about 8 percent of the time. However, during the next six months, viral load passed that threshold for a little over 1 percent of the time. In patients who had been taking antiretroviral treatment for more than five years, viral load passed that potentially infectious level only 0.03 percent of the time. 

Both this study and the Swiss statement were based on data collected from heterosexual couples.