U=U stands for Undetectable equals Untransmittable. When someone with HIV is on treatment and maintains an undetectable viral load, they cannot pass HIV through vaginal or anal sex. Large studies following thousands of couples have found zero linked sexual transmissions when viral load remains undetectable.

Can someone with an undetectable viral load transmit HIV through sex?

Based on current evidence, a person with a sustained undetectable viral load does not transmit HIV through vaginal or anal sex. Oral sex already carries very low risk, and undetectable viral load makes the risk even lower. The key is staying on treatment and keeping viral load undetectable over time.

How does someone become undetectable?

A person can become undetectable by taking antiretroviral therapy (ART) every day as prescribed. For many people, viral load drops to undetectable levels within about three to six months. Regular monitoring through blood tests confirms that the viral load has reached and remains at undetectable levels.

Does U=U mean HIV is cured?

No. U=U does not mean HIV is cured. The virus is still in the body, but at levels that are too low to measure and too low to transmit sexually. If treatment stops or adherence drops, viral load can rise again, which increases the risk of transmission.

Viral Load, U=U and What "Undetectable" Really Means

How treatment reduces HIV transmission to almost zero — and why viral load matters.

Fast answer

When someone with HIV is on treatment and reaches an “undetectable viral load,” they cannot pass HIV through sex.

U=U means Undetectable = Untransmittable. If the amount of virus in the blood is so low that it cannot be measured on a lab test, the risk of transmission is effectively zero.

This applies to vaginal and anal sex. For oral sex, transmission risk was already extremely low, even without U=U (learn more here).

HIV risk is not just about the type of sex or whether a condom was used. One of the most important factors is **viral load**, which is the amount of HIV in the blood. Modern treatment can reduce viral load to extremely low levels.

What is viral load?

Viral load is the measure of how much HIV is present in a person's blood. When someone is not on treatment, the virus can multiply quickly, leading to higher risk of transmission. When treatment is consistent, viral load can drop to the point where it is very difficult to detect.

High viral load = Higher chance of transmission
Low viral load = Much lower transmission risk
Undetectable viral load = Transmission through sex becomes effectively zero

What does “undetectable” actually mean?

Undetectable does not mean the virus is gone. It means there is so little HIV in the bloodstream that standard lab tests cannot measure it. The virus is still present, but it is not transmissible sexually.

Undetectable viral load: HIV is still present, just extremely low.

Result: No sexual transmission has been observed when viral load stays undetectable.

This is what U=U represents.

Studies have followed tens of thousands of condomless sexual encounters in couples where one partner was HIV positive and undetectable. Across those studies, there were **zero linked transmissions**.

How someone becomes undetectable

This is achieved through ART treatment (antiretroviral therapy). With daily medication, many people reach undetectable levels within about 3 to 6 months. Staying undetectable requires consistency and regular monitoring.

Requirements for U=U:

Daily ART medication with high adherence
Regular blood tests confirming undetectable viral load
Maintaining treatment without interruption

If medication stops, viral load can rise again. This is why ongoing treatment matters.

Does U=U apply to every type of sexual contact?

Yes — vaginal and anal sex pose no transmission risk when viral load is undetectable.

Oral sex was already extremely low risk (see breakdown here), so U=U only reinforces how safe it becomes.

What U=U means for anxiety and relationships

Many people panic after an encounter or obsess over symptoms like fatigue or sore throat. If a partner was on ART and undetectable, the data is clear: sexual transmission does not occur.

If you are struggling with symptom anxiety, our guide here may help: HIV Symptoms: Separating Fact From Fear.

If you’re unsure about viral load — you can calculate risk clearly

If you do not know whether a partner was undetectable or on ART, risk can still be estimated using behaviours, country prevalence, condom use and other factors. Our tool breaks that down in a personalised report using actual data.

You can also learn the basics of transmission here: HIV Risk Calculator Guide.

Get a personalised HIV risk report

Evidence based, private, and delivered to your inbox.