Effective treatment for HIV with antiretroviral drugs reduces the amount of virus in the blood to undetectable levels. NIAID-supported research has demonstrated that people living with HIV who take antiretroviral medications daily as prescribed and who achieve and maintain a durably undetectable viral load (the amount of HIV in the blood) have effectively no risk of sexually transmitting the virus to an HIV-negative partner. Thus, treatment for HIV is a powerful arrow in the quiver of HIV prevention tools. Read more about how a durably undetectable viral load can prevent HIV transmission with NIAID's fact sheet 10 Things to Know About HIV Suppression.
For nearly two decades, scientists have recognized that viral load is a key determinant of HIV transmission. Studies conducted before the availability of antiretroviral therapy (ART) revealed that higher viral loads correlate with higher rates of both sexual and perinatal transmission of HIV. Following the advent of triple-drug ART in 1996, observational studies suggested that viral loads lowered by ART were associated with reduced risk of sexual and perinatal HIV transmission. In addition, epidemiological studies showed that as the number of people in a community who are virally suppressed rises, the number of new HIV infections falls.
Findings from the large NIAID-supported HPTN 052 clinical trial, which tracked more than 1,600 heterosexual couples over 10 years, offered clear-cut evidence that consistent suppression of HIV with ART also prevents sexual transmission of the virus. HPTN 052 found that starting and sustaining treatment for HIV infection early, when the immune system is relatively healthy, essentially eliminated the transmission of HIV. No HIV transmission was observed when antiretroviral therapy consistently, durably suppressed the virus in the partner living with HIV. While some transmission events did occur in the study, new infections resulted when the partner living with HIV was not fully virally suppressed due to either having just started antiretroviral therapy, or for whom treatment no longer was working and the virus was replicating. The primary finding from HPTN 052 was a 96 percent reduction in risk of infection, which the journal Science hailed as the scientific breakthrough of the year in 2011. The HPTN 052 results, along with those of the NIAID-funded START study, helped influence the World Health Organization in 2015 to recommend that everyone living with HIV should begin treatment upon diagnosis.
Two additional studies, PARTNER and Opposites Attract, reinforced the conclusion that there is effectively no risk of sexual HIV transmission when the partner living with HIV has a sustained undetectable viral load. These studies also extended the finding to male-male couples. Findings reported in 2018 from PARTNER2, the second phase of the PARTNER study, confirmed that HIV treatment as prevention is as highly effective among gay, bisexual, and other men who have sex with men as it is among male-female couples.
To read more about the underlying science and the value of HIV treatment as prevention, see NIAID’s blog post Science Validates Undetectable = Untransmittable HIV Prevention Message.