Ever since the HPTN Study 052, we've concluded that having an undetectable HIV viral load strongly reduces the risk of HIV transmission. But how safe is condomless sex if one's viral load is undetectable?

The PARTNER Study is an ongoing large European study examining HIV transmission among mixed-status heterosexual or MSM (men who have sex with men) couples that expressly do not regularly use condoms, nor take pre-exposure or post-exposure prophylaxis. The positive partner had to have a HIV viral load less than 200 copies/mL at study enrollment.

Seven hundred seventy-six couples are currently enrolled, having already contributed data on nearly 900 couple-years and an estimated 44,400 sex acts. About 30% of the sex acts were reported to be without condoms. There were no HIV transmission events between the partners in the study, either among heterosexuals or MSM. The authors were cautious in their conclusions, but stated that the greatest estimate of HIV risk (i.e., the 95% confidence interval) was 0.45% per year and for anal sex 1% per year. Investigators speculate that these rates translate into a maximum of 5% chance of transmission over a 10-year period.

While it's important to be mindful that condom-free sex still carries with it the risk of other sexually transmitted infections, this study's conclusions are profoundly important. PARTNER confirms the simple fact that HIV treatment is prevention. In the real world where condom use is at best imperfect, treating positives can prevent HIV transmission to their same- or different-sex partners.

PARTNER's data also have important implications in counseling partners desiring pregnancy, especially in places where assisted pregnancy or sperm washing are not available. There are also possible downstream implications for legal systems that criminalize sex between positives and negatives, an issue pioneered by Swiss governmental statements. Along with HPTN 052, this study lends the strongest support to treatment as an essential component to HIV prevention programs.

Which other studies presented at CROI 2014 will have lasting impact? Read more of Dr. Llibre and Dr. Young's top picks.