U=U FAQ

U=U is shorthand for ‘Undetectable equals Untransmittable”, language used by the HIV community to communicate that people who are HIV+ with a suppressed viral load are unable to transmit the virus to their sexual partners.

When we have a viral infection of any kind (like HIV, or HSV, or COVID-19) we have a certain amount of virus in our body. This is called our ‘viral load’. If we are on medication, or we have been vaccinated against a virus, our immune system will suppress the number of viral copies in our body. A virus can be so well suppressed that we barely have any in our body, we don’t get sick at all, and we can’t pass it on to anyone else.

Viral load is measured in copies of virus per milliliter of blood (or: the number of fish per gallon of pond water). When it comes to HIV, the number of copies we can have in our blood before we are infectious to other people is usually described as 200 per milliliter (although some studies have found U=U at even higher thresholds - see below). That means that if an HIV+ person has fewer than 200 copies/mL of HIV in their body, they simply do not have enough virus in their body to infect another person, even if they were to have the most high-risk sex possible. This also makes ‘undetectable’ a bit of a misnomer - even if we are able to detect the virus in people who have achieved this level of viral suppression, they are still unable to transmit it.

The many large-scale studies of U=U have all come to the same conclusion. These studies generally work by taking large groups of couples where one person in the couple has HIV and the other person does not (“serodiscordant”). They give the HIV+ person HIV medication (antiretroviral treatment, or ART) to lower their viral load until it is undetectable. They keep measuring their viral load throughout the study. They also continually test the HIV negative person for HIV. They follow these couples over a long period of time to see if the HIV negative half of the couple contracts HIV from their HIV+ partner. The main studies include:

HIV Prevention Trials Network (HPTN) 052

This study followed 1763 couples (97% heterosexual) from the United States, Botswana, Brazil, India, Kenya, Malawi, South Africa, Thailand, and Zimbabwe. During the whole study (which ran from 2005 to 2011) there was not a single instance of HIV transmission where the HIV+ partner had a viral load below 400 copies/mL.

PARTNER Study

This study enrolled 1166 couples from 14 countries in Europe. This included heterosexual couples where the man was HIV+, heterosexual couples where the woman was HIV+, and male homosexual couples. During the 4 years it ran, totally approximately 58,000 acts of condomless sex, there were no confirmed transmissions of HIV from the positive partners (who all maintained viral loads under 200 copies/mL) to the negative partners. Among the people who started the study HIV negative, 11 became HIV positive – but genetic analyses showed that none of those transmissions could be traced back to the HIV+ partner with the suppressed viral load, and most of those participants admitted they also had condomless sex with other people besides the partner they were enrolled in the study with.

Opposites Attract

This study followed 348 male homosexual couples in Australia, Brazil, and Thailand. Over four years (and nearly 17,000 acts of condomless sex) there were only 3 new infections, and again these were not genetically linked to the enrolled HIV+ partner.

Spanish Study

A long-term Spanish study found that over 417 couple years (EXPLAIN) and 7000 acts of condomless sex, there were no transmissions of HIV among 144 serodiscordant couples where the positive partner was on ART. The same researchers conducted another study where there were no transmissions of HIV among 199 serodiscordant couples, totalling 7600 risky sexual encounters, where the HIV+ partner was on ART.

The “Swiss Statement”

The original paper which first declared an undetectable viral load eliminates transmission risk was published in 2008 in a Swiss medical journal. This summarized several long-term studies representing 548 serodiscordant couples with no instances of transmission, to argue that under 400 copies/mL, an HIV+ person is not infectious.

Scientists never speak in certainties or absolutes. They will never say something is 100% or 0% possible, even when all the evidence suggests as much. We know, for example, that there has been some suspicion around the language used by the CDC, who state that there is “effectively no risk” of HIV transmission when the person has an undetectable viral load. “Effectively no risk” may seem less certain than “no risk”, but as the CDC explains, this language “was selected to reflect that while it is not possible to statistically rule out a non-zero risk, all evidence to date suggests that it is not realistically possible to sexually transmit HIV while the person with HIV remains undetectable or virally suppressed.”

It is also important to remember that no sexual health system has ever or can ever provide a 100% guarantee of anything. Even if HIV+ performers are excluded from performing, there is always the chance that someone who tested HIV negative on their last test could theoretically contract HIV in between tests, either on or off set, and then go to work with someone and transmit the virus. In many ways, then, knowing someone is HIV+ and undetectable offers more certainty of safety than believing someone is HIV negative based on their last test. That’s because HIV is most contagious when someone is first infected and doesn’t know it yet. The HIV viral load skyrockets when someone first gets infected, (i.e. they have LOTS of copies per milliliter or blood in their body), making them the most infectious before they are even aware they have contracted the virus.

Viral ‘blips’ are temporary increases in an HIV+ person’s viral load (the amount of virus in their blood). Viral blips can happen to people because they get sick or receive a vaccine, which preoccupies their immune system, or because they miss a dose of their meds, or for no reason at all. Viral blips are common, and that’s a good thing because it means that the occurrence of viral blips has been factored into the many studies of undetectable HIV. A ‘blip’ refers to any fluctuation in someone’s viral load, so if someone is on HIV medication, their ‘blips’ still occur under the threshold of being untransmittable.

For a ‘blip’ to occur that would make someone infectious again, then they would no longer be undetectable on their test. Someone would have to stop taking their meds for a long time in order to become infectious again. They could not become infectious within the time period between tests.

HIV Viral load testing in the studies measured down to 200 copies of the virus, demonstrating that a viral load of less than 200 copies equaled untransmittable. Viral load testing in clinical practice measures down to 20 copies in most cases (some labs may use less than 50 copies as undetectable). Therefore, a “blip” of greater than 20 copies but still less than 200 copies would be considered untransmittable.

Because these studies were done under real-world conditions, they take into account real-world concerns. Like blips, the presence of other STIs like gonorrhea have been factored into the studies of U=U and have found that if someone has an undetectable viral load, the presence of another STI does not change the fact that they cannot transmit HIV. Everyone is vulnerable to STIs, and it is important for HIV+ performers to have access to an effective testing system for work so they can also avoid contracting STIs.