HIV: undetectable = untransmittable? Yes. But the news is even better.

I vaguely remember the first time I heard someone say “I am a person living with AIDS.” It must have been the late 1980s or early 1990s, because even as a kid or a teen, the turn of phrase struck me as aspirational, if not an overt euphemism. “Okay, but in reality, that person is dying of AIDS” is the awful thought I remember thinking.

In the mid-to-late 1990s, though, the possibility of sustainably “living with HIV” became a reality. It was no longer wishful thinking. People with access to powerful combinations of antiviral drugs could now escape the virus’s previously intractable progression to AIDS and its devastating endgame marked by a series of horrific and deadly infections.

That transition occurred before I entered medicine. But at some point during my medical training, I became aware of another sea change. This one was a bit more of an open secret, though. People who had undetectably low viral levels (by virtue of taking the right combination of antivirals for long enough) were not spreading HIV.

This would seem to be the type of good news that you’d expect people to be shouting from the rooftops. But it wasn’t. The way I remember it, there was a sense in the mainstream medical community that perhaps the public should not be clued into this. The worry was that if people got the idea that HIV could not be spread by people who had undetectable levels of virus, it might lead them to abandon safe sex practices.

Fortunately, that viewpoint fell out of favor over time.

In 2008, researchers reported that those with undetectable viral levels were not spreading the virus. Out of that and other similar work, the mantra “undetectable = untransmittable (or “U=U”) was born. (A nice review of this concept published in 2019 was co-authored by one Dr. Anthony Fauci).  By the 2010s, public health had come around to an important understanding; patients would be further motivated to maintain reliable adherence to their antiviral drug regimens if it meant they could be unburdened from the fears that they might spread the virus to others. In practice, antivirals decrease transmission among couples by 96% or more.

Now, researchers have announced yet another step in the right direction. While U=U remains the goal, data now indicate that even “getting close” is good enough. In a study of studies published in The Lancet, researchers found that people with HIV with viral levels that were detectable but under a certain threshold (1,000 viral copies per milliliter, compared to levels ranging from 30,000 to 500,000 copies in those not being treated) were exceedingly unlikely to transmit the virus. (At under 1,000 copies per milliliter, the real risk is probably zero, but I’ll leave the caveats aside for now.) This is good for many reasons. One is that it means that cheaper yes/no tests can be relied on to tell people if there is any chance they have high enough viral levels to spread the virus by means of unprotected sexual contact.

The story of HIV antivirals has to be one of modern medical science’s great triumphs. For people at high risk of contracting HIV, taking these drugs prophylactically (known as PrEP) reduces infection rates by around 99%. These drugs reduce the progression from HIV infection to full-blown AIDS. Lastly, they stop transmission from infected individuals. While an HIV vaccine could still save many lives and prevent suffering, it’s remarkable that antiviral pills—many of which are inexpensive—can now achieve most of what a vaccine would hope to accomplish.