When people newly diagnosed with HIV come for an appointment with Anu Hazra, M.D., he says they often ask the following questions: Will I ever be able to be intimate with anyone, again? Will anyone want to have sex with me? Am I going to be alone for the rest of my life?

This is why the power of the message of undetectable equals untransmittable, or U=U, is so meaningful, says Hazra, who is an infectious disease specialist at University of Chicago Medicine. That power exists because of modern antiretroviral therapy (ART) that, when taken as prescribed, can suppress viral levels down to what’s called undetectable levels.

“We have clear evidence, hard evidence, that really shows that suppressed viral load cannot transmit HIV sexually—at all, period, full stop, no caveats,” Hazra tells TheBody.

What Does U=U Mean?

The nonprofit Prevention Access Campaign worked with people living with HIV and researchers to develop the slogan “undetectable equals untransmittable,” or U=U. The slogan, which the organization launched in 2016, was intended to easily communicate the scientific evidence showing that HIV cannot be transmitted from having sex with people who are on ART with viral loads below 200 copies/mL.

What Does Undetectable HIV Mean?

Blood tests measure the quantity of virus contained in the blood of a person living with HIV. The results measure the amount of “copies” of the virus in a milliliter of blood, says Hazra.

When ART suppresses HIV, the medicine maintains viral loads that are so low that lab tests can no longer “see” the virus in a person’s blood. When this occurs in someone previously diagnosed with HIV, that’s called undetectable HIV, which is often simply called “undetectable.”

“[But] all the studies proving U=U are using the viral load threshold of 200 copies/mL because that was the best the machinery could actually detect at that time,” Hazra says. Just as ART has improved over time, so have HIV tests and, as a result, the definition of undetectable has shrunk.

This can add confusion for both people living with HIV and some clinicians because undetectable can refer to different viral-load thresholds. In the context of U=U, undetectable means anything below 200 copies/mL, which clinicians also call viral suppression. In the context of an HIV viral-load test today in the U.S., undetectable typically means below 20 copies, but different tests may have higher detectable thresholds.

Whether a person has viral loads below 200 copies/mL or 20 is irrelevant in terms of what U=U means, however. Sexual transmission of HIV cannot occur in anyone with viral loads below 200 copies/mL, so any test result that’s below 200 copies/mL—detectable or undetectable—means the probability of sexual transmission is zero.

If you’re on a dating app, you may see someone calling themself “positive undetectable,” which means they’re living with HIV and their virus is undetectable and untransmittable.

Research Behind U=U

Four clinical trials serve as the main evidence behind U=U:

  • HPTN 052 followed over 1,700 sero-different couples, most of whom were heterosexual (97%), at 13 sites in nine countries between 2010 and 2015. The study found that taking ART led to a 93% lower likelihood that an HIV-negative partner would acquire the virus. While new HIV cases did occur during the study, two factors caused them. Some cases were due to the partner with HIV not being virally suppressed. Other cases were due to the partner who was seronegative acquiring the virus from someone else, as revealed through genetic comparisons of the couple’s HIV strains. Zero new HIV cases occurred among couples when the person living with HIV was virally suppressed.

  • PARTNER 1 followed more than 1,100 sero-different couples—a mix of heterosexual (62%) and gay and bisexual men (38%)—at 75 sites in 14 European countries between 2010 and 2014. Eleven new HIV cases occurred during the study, but genetic testing revealed that none of the transmission occurred between couples.

  • PARTNER 2 extended the follow-up period of 477 gay couples from the PARTNER 1 study until 2017, and 495 new gay couples were recruited and monitored between 2014 and 2017. Again, genetic testing showed that people acquired the virus outside of their relationship—none occurred between couples when HIV was virally suppressed.

  • Opposites Attract followed about 350 male gay couples at 15 clinics in three countries between 2012 and 2016. Three new HIV cases occurred during the study, but, yet again, none were genetically linked, meaning the partner without HIV acquired the virus from another person.

Taken together, these studies prove that a person without HIV cannot acquire the virus through sex with a person who is on ART and virally suppressed (below 200 copies/mL). In other words, if you’re someone who doesn’t have HIV, there’s absolutely no reason to fear or avoid having sex with someone who is living with undetectable HIV—because you can’t get the virus from them.

Is Being Undetectable the Same as Being HIV Negative?

Neither being virally suppressed nor being undetectable is the same as being HIV negative. HIV negative is a term that applies to people who’ve never tested positive for the virus. “Viral suppression” and “undetectable” are terms that apply only to people previously diagnosed with HIV but who now take ART as prescribed and lower their viral load to 200 copies/mL or fewer.

People with HIV who reach undetectable viral loads still have viral particles in some of the body’s cells, which are typically referred to as the HIV reservoir. Taking ART as prescribed does not cure someone of HIV, but it prevents the virus from spreading to other cells in the body and helps them live a long and healthy life.

How Does Someone Become Undetectable, or Untransmittable?

People can become undetectable (which, by definition, means untransmittable), by taking HIV medicine (ART) according to their doctor’s instructions.

Today, there are many regimens that allow a person with HIV to become undetectable. It is important to note that your doctor will help you choose the best formulation that fits your specific need. According to U.S. clinical guidelines, a wide variety of drug combinations exist, including:

  • Bictegravir, tenofovir alafenamide, and emtricitabine—marketed as Biktarvy.

  • Dolutegravir, abacavir, and lamivudine—marketed as Triumeq.

  • Dolutegravir and lamivudine—marketed as Dovato.

Many other regimens can help people become undetectable, and your doctor will evaluate your specific medical needs to choose the best fit for you.

How Long Does It Take to Become Undetectable?

Once a person has been diagnosed with HIV, clinicians will prescribe ART and monitor viral loads to make sure the medicines are working. With modern ART formulations, Hazra says, “we’re able to see really sharp declines in viral loads much faster than we had with older regimens.”

He says that the majority of his patients who begin ART after a new diagnosis are able to achieve viral loads below 200 copies/mL within a matter of two to three weeks. If a person has very high viral loads, as occurs during the acute stage of HIV, Hazra says that it can take four to six weeks to achieve undetectable viral loads.

How To Prevent Sexual Transmission of HIV Until You Are Undetectable

While you’re waiting to become undetectable, a variety of methods exist to help prevent transmission through sex, such as using condoms or asking your partner to take PrEP. Once your levels reach undetectable, having condomless sex will not transmit HIV, although you may be exposed to other sexually transmitted infections.

What Are Blips, and How Do They Affect U=U?

After someone has achieved undetectable status by taking their ART as prescribed, Hazra says that clinical guidelines recommend monitoring viral loads every three months. Sometimes, those results can jump up to detectable values, which are called “blips.”

“What’s still poorly understood is why some people have blips and some people don’t have blips,” says Hazra. Most blips, he says, remain under 200 copies/mL and usually return to undetectable in the next test. In those cases, U=U still applies, and these temporary upticks in viral load are still incapable of causing sexual transmission of HIV.

Sometimes blips can go above 200 copies/mL. Hazra says that if this occurred, he’d call his patient to make sure they didn’t miss an ART dose or check to see if they were taking any supplements that might interact with their medication.

A 2023 review concluded that for viral loads below 1,000 copies/mL, there is “almost zero risk of sexual transmission of HIV.” That means that blips above 200 copies/mL don’t carry a zero risk of sexual transmission like U=U, but they still have a very low likelihood. For blips above 200 and below 1,000 copies/mL, Hazra says he would advise patients that there could potentially be concern for sexual transmission until the next test result.

If a test result comes in above 1,000 copies/mL, Hazra says that’s not likely to meet the definition of a blip. “Usually that’s in the setting of some folks not being able to take their medication or not being able to be engaged in care anymore, or other issues that impact your ability to take the medication.” It’s also possible this could represent an issue with the ART regimen. At this point, a person is no longer virologically suppressed, says Hazra, and there is potential for sexual transmission (as well as other health concerns). Therefore, he’d recommend the person come back to the clinic for further evaluation.

Does Someone Have to Disclose Their HIV Status if They Are Undetectable?

Disclosing your HIV status is a highly personal decision, especially given the extent to which the virus is still stigmatized. You can be sure that if your viral loads are undetectable, you are not putting your sexual partners at any risk of acquiring the virus.

However, your decision to disclose may depend on state HIV criminalization laws, which discriminate against people with HIV. Some states consider not disclosing your HIV status to a sex partner a crime. Many criminalization laws don’t consider intent or actual risk, so whether you’re on ART and undetectable might not matter. Under those laws, people can be arrested and prosecuted even when having sex poses no risk. Be sure to review laws in your state, or states you may be traveling to.

U=U in Pregnancy and Childbirth

Although maintaining an undetectable viral load reduces the likelihood of HIV transmission through other modes such as from a birthing parent to a child, U=U only directly applies to sexual transmission. There aren’t enough data to conclude that with any other mode of transmission, HIV would be rendered completely untransmittable with undetectable viral loads. However, we do have some encouraging data for pregnant people.

In France, for example, researchers tracked more than 14,000 pregnant women with HIV to see how ART impacted perinatal transmission. Transmission to infants was highest when mothers were untreated and lowest when they’d started ART prior to conception. Out of almost 5,500 births from women who began ART at conception and were undetectable near delivery, zero infants acquired HIV. Out of nearly 2,400 births from mothers who maintained viral loads below 50 copies/mL during their first trimester, zero infants acquired HIV.

However, over the entire study, viral loads above 50 copies/mL did carry a greater-than-zero likelihood of transmission. It’s also important to note that none of the women breastfed their infants, and all of them had free access to ART and monthly viral-load testing.

Hazra says that if a pregnant person’s viral load is below 1,000 at the time of delivery, then a baby may not even need HIV prophylaxis after birth. “[The] importance of being undetectable—even under 1,000—at the time of delivery can really impact our concerns about vertical transmission to the infants,” he says.

Breast- and chestfeeding is another matter. “We don’t have data to show that U=U can be applied directly to breast- and chestfeeding,” says Hazra. If a pregnant person is virally suppressed during pregnancy and while breastfeeding, the likelihood of an infant acquiring HIV through breast milk is less than 1%—but not zero. Hazra points out that current clinical guidelines state that choosing to breastfeed is a viable option for someone with undetectable levels.

Despite U=U not directly applying to a pregnant person with HIV and their child, taking ART as prescribed and maintaining undetectable viral loads helps reduce both perinatal and breastfeeding transmission.

Key Takeaways

Here are 13 takeaways concerning U=U:

  • Sexual transmission of HIV cannot occur when involving someone with viral loads below 200 copies/mL.

  • In the context of U=U, undetectable means anything below 200 copies/mL, which clinicians also call “viral suppression.”

  • In the context of an HIV viral-load test today in the U.S., “undetectable” typically means below 20 copies/mL, but different tests may have higher detectable thresholds.

  • Any test result that’s below 200 copies/mL—detectable or undetectable—means the probability of sexual transmission is zero.

  • If you’re someone who doesn’t have HIV, there’s absolutely no reason to fear or avoid having sex with someone who has undetectable HIV—because you can’t get the virus from them.

  • Several clinical trials involving thousands of people provided incontrovertible proof that U=U is 100% accurate. The science is clear and undeniable.

  • People can become undetectable by taking HIV medicine according to their doctor’s instructions.

  • Modern ART formulations can help most people lower their viral loads to an undetectable level within weeks of initiation.

  • Blips in viral loads occur in some people, but researchers are not completely clear why. If blips are below 200 copies/mL, there is no risk of HIV transmission through sex. If a blip exceeds 200 but is below 1,000 copies/mL, the risk of sexual transmission is “almost zero” but not zero, so consider other prevention strategies until viral loads decrease.

  • A viral load measurement above 1,000 is likely not a blip and may represent a treatment issue that requires medical consultation. Sexual transmission of HIV can occur at this viral load, so consider alternative prevention strategies until viral loads decrease.

  • Disclosing your HIV status to a sex partner is a personal decision, but know that some states consider it a crime to not do so.

  • U=U only directly applies to sexual transmission, but maintaining an undetectable viral load reduces the likelihood of other HIV transmission modes.

  • Maintaining undetectable viral loads during pregnancy and after significantly reduces the risk of perinatal transmission and transmission during breast- or chestfeeding, but not to zero.

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