A two-year study gives scientific credence to what many have long suspected: HIV positive guys who are on treatment and have an undetectable viral load are not giving HIV to their partners, not matter how hard they try. The study strengthens the belief that “treatment as prevention” is one of the most effective ways to stop new infections.

The two-year study, presented at the Conferences on Retroviruses and Opportunistic Infections this week, showed as low a risk of infection as you can find in research. As reported by AIDSMap:

When asked what the study tells us about the chance of someone with an undetectable viral load transmitting HIV, presenter Alison Rodger said: “Our best estimate is it’s zero.”

Participants in the study were couples, gay and straight, in which one partner had HIV and the other did not. They were selected because they had sex without condoms at least some of the time, and the negative partner was not on PrEP (taking the drug Truvada to prevent infection).

The couples kept themselves very busy sexing it up in the name of science: the study reported 16,400 sexual acts among the gay couples, including being on top, being on bottom, oral sex, and plenty of “ejaculate” on and in the bodies of participants (imagine conducting those interviews with the couples).

None of the negative subjects were infected by their positive partners, although a few negative partners got infected by someone outside the relationship, which was determined by genetic testing of the HIV strain. Those guys then had some explaining to do, don’t you think? Or, perhaps, not.

Of course, you have to know if you’re positive in the first place for treatment to make a difference. At least 20% of those with HIV in the U.S. don’t know they have it. “The keys to keeping everyone healthy is for you to be regularly tested for HIV, and if you are positive, to take advantage of effective treatments,” said Raymond C. Martins, M.D., of Whitman-Walker Health in Washington, DC. “For gay men who are negative, please remember that the higher risk comes from those men who do not know their HIV status and might in fact have high levels of infectious virus.”

The jury is still out on whether or not studies like this will affect HIV stigma, but one thing is certain: our friends with HIV who are on successful treatment are definitely doing their part to stay healthy — and protect the rest of us, too.

Way to go, poz dudes.