The PARTNER study, a large observational study from multiple European sites, is attempting to estimate the risk of “condomless sex” in serodiscordant gay and straight couples in which the HIV-positive partner has an undectable viral load. They found NO transmissions from the positive partner to the negative partner, even with receptive anal sex (“bottoming”) with ejaculation, the sexual act with the highest risk of transmission.  These findings back up the results of HPTN 052, which found a 96% reduction in sexual transmission (mostly heterosexual) when the positive partner was on ART (and probably NO transmissions when the positive partner had an undetectable viral load).

Does this mean that discordant couples (or “serodifferent couples,” as the author called them) can dispense with condoms altogether?  Not necessarily. While the transmission rate was zero in this study, the presenter made a point of discussing the uncertainty around that number.  She pointed out that while their best estimate for the transmission rate is zero, they can’t exclude the possibility, with 95% certainty, that the true overall transmission rate was up to 4% over 10 years, and that the true transmission rate for receptive anal sex with ejaculation was up to 10% over 10 years. This is basically a statistical way of saying that you can’t prove a negative.  As the study accumulates more couples, the statistical uncertainty will diminish, and if they continue to see no transmissions, they will be able to say with greater certainty that the estimated risk is zero.

In the meantime, seronegative MSM should understand that there is still a small possibility of becoming infected by a partner with an undetectable viral load.  This could happen in one of two ways: (1) since we check the viral load every few months rather than every day, the viral load may have increased after the last test, or (2) there may be detectable virus in semen even though it’s undetectable in blood.  We know this blood/semen discordance occurs, especially in people who have not been on ART for very long. Seronegative men should still consider using condoms with a partner who has an undetectable viral load, especially for the highest risk activity: receptive anal sex with ejaculation.  They should also remember that ART protects them from HIV but not against other sexually transmitted infections.

Caveats aside, the PARTNER study provides further evidence that treatment really is prevention.