No. A person living with HIV on antiretroviral therapy (ART) with an undetectable HIV viral load in their blood for at least six months cannot transmit HIV through sex.

Viral load refers to the amount of HIV in a blood sample of a person living with HIV. Generally, the higher the viral load, the more likely you are to transmit HIV.  Both "undetectable" and "viral load suppression" are extremely low measurements of HIV that are virtually the same. The terms are often used interchangeably. 

_ Undetectable viral load:  ART can reduce a person’s viral load to the point where it is so low (usually under 20 or 40 copies/ml in Vietnam) that it cannot be detected by measurements. This is called ‘having an ‘undetectable viral load,’ which prevents the sexual transmission of HIV while simultaneously improving the health of a person living with HIV. It does not fully clear the virus from the body or cure someone of HIV.  Excellent adherence, or taking ART as prescribed, is important to maintain an undetectable viral load.   

_ Viral load suppression: When ART suppresses a person's viral load to less than 200 copies/ml, this is called being "virally suppressed." Being virally suppressed prevents the sexual transmission of HIV while simultaneously improving the health of a person living with HIV. Studies show that when a person is virally suppressed they cannot transmit HIV to sexual partners. Excellent adherence, or taking ART as prescribed, is important to maintain viral suppression. 

For the purposes of the U=U campaign, the term "undetectable" is used synonymously with the term "virally suppressed."

Even when viral load is undetectable, HIV is still present in the body. The virus lies dormant inside a small number of cells in the body called viral reservoirs. When therapy is halted by missing doses, taking a treatment holiday, or stopping treatment, the virus emerges and begins to multiply, becoming detectable in the blood again. This newly reproducing virus is infectious. It is essential to take medications daily as directed to achieve and maintain an undetectable status.

Four studies — HPTN 052, PARTNER, PARTNER 2 and Opposites Attract — followed thousands of couples across multiple countries. In these studies, one partner was HIV-positive and the other HIV-negative. The studies looked at the risk of HIV transmission when the HIV-positive partner was on treatment with an undetectable viral load (defined as less than 200 copies/ml). No other HIV prevention interventions, such as condoms or PrEP, were used. 

These four large studies together documented nearly 130,000 heterosexual and male-male sex acts across thousands of heterosexual and male-male couples. Despite these couples not using condoms or other prevention interventions, there were ZERO transmissions of HIV from the positive partner to the negative partner when the positive partner was stably virally suppressed.

The scientific evidence is clear: people who take HIV treatment daily as prescribed and achieve and maintain an undetectable viral load have no risk of transmitting HIV through sex.

If you have HIV, take antiretroviral medicines as prescribed by your health care provider. After you start your medicine, your provider will take blood samples for a viral load test, to determine the level of HIV virus in your blood. It can take one to six months of taking your antiretroviral therapy as prescribed every day before your viral load becomes undetectable (<200 copies/ml). Once your viral load has been undetectable for six months you cannot transmit HIV to your sex partners. Daily adherence to medication is essential to keeping your viral load suppressed

Nearly everyone who starts ART finds a drug regimen that works within six months. About one out of six people will need additional time to find the right treatment due to tolerance and adherence issues. Adherence to treatment and regular viral load monitoring are essential to maintaining an undetectable viral load.

In Vietnam, viral load testing is recommended 6-months after starting ART and every 12 months thereafter for people who have a stable undetectable viral load.

It means that if you are undetectable and stay on HIV treatment, you are likely to be much healthier than if you were not on treatment and no longer need to be constrained by fear of transmitting the virus to others during sexual experiences. If you want to stop using condoms, it is important to discuss this carefully with your partners and ensure they are also comfortable with the decision. It is important to remember that while an undetectable viral load will protect your partners from your HIV, it does not protect them or you from other sexually transmitted infections (STIs) or prevent pregnancy.  

You need not be constrained by fear of acquiring HIV while your sexual partner is undetectable. If you want to stop using condoms, it is important to discuss this carefully with your partner and ensure they are also comfortable with the decision. It is important to remember that while an undetectable viral load will prevent transmission of HIV, it does not protect you or them from other sexually transmitted infections (STIs) or unwanted pregnancy.

_ Get an HIV test. A positive test is an opportunity to treat HIV, stay healthy and prevent HIV transmission. A negative test gives you the chance to discuss ways to stay negative, like using condoms, taking daily PrEP or taking emergency PEP.
_ Condom use. Having an undetectable viral load prevents HIV transmission but it does prevent other sexually transmitted infections (STIs) like syphilis, gonorrhea, and chlamydia. Condoms protect against HIV, other STIs, and unintended pregnancy.
_ Pre-exposure prophylaxis (PrEP) is for HIV-negative persons at substantial risk for HIV. PrEP is not medically necessary for the HIV-negative partner when the HIV-positive partner has been undetectable for more than six months and stays adherent to their treatment regimen. However, the HIV-negative partner may want to consider PrEP if: 
a, The HIV-positive partner does not take ART every day as prescribed. 
b, The HIV-negative partner is having sex with other partners and 
c, The HIV-negative partner feels more secure in their sex lives with the added protection of PrEP.
_ Get tested regularly for other STIs. STIs may not show symptoms, but they can increase an HIV-positive person’s viral load, or make it easier for the virus to enter an HIV-negative person’s body.
_ Post-exposure prophylaxis (PEP) is medication to prevent HIV that can be taken after a high-risk exposure to HIV. PEP must be taken as soon as possible after the exposure, and always within 72 hours. 

Knowing how an undetectable viral load prevents HIV transmission may be especially useful for people wishing to conceive a baby without using alternative insemination practices. An undetectable viral load also dramatically reduces the risk of vertical transmission - during pregnancy or breastfeeding.

U=U does not apply to HIV transmission through needle sharing. There is not yet enough research to draw a conclusion.