Experts at a U.K. conference confirmed that effective antiretroviral (ARV) therapy “significantly reduces” the risk of HIV transmission. However, according to a report from aidsmap, researchers and community leaders at the gathering avoided speaking in absolute terms. For example, they did not say that risk of spreading the virus is nonexistent in HIV-positive people with undetectable viral loads and no other sexually transmitted infections (STIs). They also pointed out that important questions remain unanswered.

Earlier this year, in a document widely known as the “Swiss Statement,” four renowned experts concluded that HIV-positive people receiving ARV therapy with undetectable viral loads and no STIs are “sexually noninfectious.” This claim, however, has since been the subject of much debate.

At a British HIV Association meeting earlier this month, Bernard Hirschel, MD, one of the four Swiss Statement authors, reiterated that several key studies indicated that HIV-positive people with undetectable viral loads were not transmitting HIV to their HIV-negative partners. However, aidsmap reports, he conceded that in medicine “you should never say never” and also pointed out that all of these studies involved heterosexual couples.

Studies also indicate that undetectable in the blood does not always mean undetectable in genital secretions. At the U.K. conference, Steve Taylor, MD, of the University of Birmingham, Alabama, said that an undetectable viral load among men “almost always” means undetectable HIV in semen. In women, however, studies suggest that up to 30 percent of women with undetectable HIV in their blood have detectable HIV in their genitals.

Taylor also commented that very little information is available from studies exploring the effect of ARV therapy on viral loads in the rectums of men and women. The one study that has explored this, aidsmap reports, found that HIV was still detectable in this area, even among people on effective HIV treatment.