Undetectable viral load and HIV transmission

Mareike Günsche | www.aspect-us.com

Key points

  • Effective HIV treatment reduces the amount of HIV in body fluids.
  • Once the amount of HIV in body fluids is reduced to an ‘undetectable’ level, HIV cannot be passed on during sex.
  • This is what is meant by the slogan 'Undetectable equals Untransmittable' ('U=U').

The amount of HIV found in body fluids is called viral load. Taking effective HIV treatment (antiretroviral therapy) suppresses the amount of HIV in body fluids. This reduces viral load. Viral load is measured in units called ‘copies’ and tests show these results as ‘viral copies per millilitre of blood’ (copies/ml). With effective HIV treatment, the number of copies becomes so low that these viral load tests are unable to detect any HIV or can only find a tiny trace. Doctors call this ‘virological suppression’ but it is often known as ‘having an undetectable viral load’ or ‘being undetectable’.

Having an undetectable viral load means that there is not enough of the virus in body fluids to pass on HIV during sex, including oral sex, vaginal sex, and anal sex. This is sometimes referred to as 'Undetectable equals Untransmittable' or 'U=U'.

Viral load and the risk of transmission

Viral load changes over time.

  • The first few weeks after someone gets HIV, their viral load is usually very high. A typical high viral load is between hundreds of thousands to several million copies/ml. There is a high risk of passing on HIV at this point.
  • A few weeks after someone gets HIV, their viral load usually drops. A typical viral load in someone not taking treatment may be 50,000 copies/ml or higher. There is still a considerable risk of passing HIV on.
  • After starting HIV treatment, viral load usually falls quickly. Within three to six months, most people who take their treatment as prescribed will have an undetectable viral load. An undetectable viral load needs to be maintained by taking effective treatment as prescribed, without regularly missing doses.

If your viral load is not undetectable, but is below 1000 copies/ml, this can be called a ‘suppressed’ viral load. With a suppressed viral load there is almost zero risk of HIV transmission during sex.

How do I know if my viral load is undetectable?

Your viral load is measured using a blood sample. This might be taken from your arm or from a finger prick. A result of anything less than 200 copies/ml means there is zero risk of HIV transmission during sex.

Some tests are able to measure very low numbers of copies. Using these tests, you might be told that you have a viral load of 50 or 150 copies per millilitre of blood. As long as this number is under 200, you can think of your viral load as undetectable. These tests are usually based on a blood sample taken from your arm.

The way your result is written will depend on the type of test that was used.


An undetectable viral load test result means the amount of HIV was so low that the test was unable to detect any HIV or could only find a tiny trace. This result might also be written as ‘not detected’ or ‘target not detected’. There is zero risk of HIV transmission during sex.

Below the limit of detection

You might be given a result that reads ‘below the limit of detection (<LOD)’ or ‘below the lower limit of quantification (<LLOQ)’. Whether this means you are undetectable depends on the test you were given. If the test had a limit of 200 or less (which is usually the case if the blood sample was taken from your arm), then being below the limit of detection means there is zero risk of passing on HIV during sex.

If you get this result and the test has a limit that is somewhere between 200 and 1000 copies/ml, then your viral load is ‘suppressed’ and there is almost zero risk of passing on HIV during sex.

You can ask your healthcare provider which type of test was used.


You might be given a result that reads ‘detected’ followed by a number of copies. For example, ’detected, below 50 copies/ml’. As long as the number of copies is 200 or less, your viral load is low enough to be considered ‘undetectable’ and there is zero risk of HIV transmission during sex. If the number is below 1000, then there is almost zero risk.

Sometimes a ‘detected’ result won’t give a specific number. This result is sometimes called ‘detected, not quantified’.  This result almost always means your viral load is less than 1000 copies/ml. For this reason, a ‘detected not quantified’ test result usually means your viral load is either suppressed or undetectable. Your healthcare provider might be able to give more detailed information about the test you were given.

Number of copies

Matthew Hodson talks about Undetectable equals Untransmittable (U=U).

Some tests can tell you exactly how many copies of the virus are in your blood. As long as this number is under 200 copies/ml, there is zero risk of HIV transmission during sex. If the number is under 1000 copies/ml, there is almost zero risk of HIV transmission.


This result means that you have a detectable viral load that is over 1000 copies/ml. This means there is a risk of passing HIV on during sex.

Above the upper limit of quantification (>ULOQ)

This means you have a high viral load of above one million or ten million copies/ml. There is a high risk of HIV transmission with a viral load above this level.

Isn’t the viral load in semen, vaginal fluids or rectum more important than viral load in blood?

Viral load in blood and in other body fluids is usually very similar. If HIV is undetectable in blood, it’s likely to be undetectable elsewhere. Occasionally people have undetectable HIV in their blood and have low levels of HIV in other body fluids, but very rarely at infectious levels.

What’s a viral load blip?

A viral blip is a short-term increase in viral load in someone who usually has an undetectable viral load. When a blip happens, viral load increases from undetectable to a low but detectable level before becoming undetectable again on the next test. Usually, a viral load blip is not a cause for concern, and sometimes it can just be because of a problem with the test. However, a blip can indicate a problem if it happens around the same time as missed or late doses of medication, or if the viral load is detectable on two tests in a row.

Does HIV treatment always work?

HIV treatment doesn’t always work straight away. About one in six people find that they can’t reach an undetectable viral load using their first treatment regimen, or their treatment stops working in the first year.


viral load

Measurement of the amount of virus in a blood sample, reported as number of HIV RNA copies per milliliter of blood plasma. Viral load is an important indicator of HIV progression and of how well treatment is working. 


undetectable viral load

A level of viral load that is too low to be picked up by the particular viral load test being used or below an agreed threshold (such as 50 copies/ml or 200 copies/ml). An undetectable viral load is the first goal of antiretroviral therapy.

Undetectable = Untransmittable (U=U)

U=U stands for Undetectable = Untransmittable. It means that when a person living with HIV is on regular treatment that lowers the amount of virus in their body to undetectable levels, there is zero risk of passing on HIV to their partners. The low level of virus is described as an undetectable viral load. 

sexually transmitted infections (STIs)

Although HIV can be sexually transmitted, the term is most often used to refer to chlamydia, gonorrhoea, syphilis, herpes, scabies, trichomonas vaginalis, etc.

detectable viral load

When viral load is detectable, this indicates that HIV is replicating in the body. If the person is taking HIV treatment but their viral load is detectable, the treatment is not working properly. There may still be a risk of HIV transmission to sexual partners.

During the second year of treatment, the chance of it stopping working is about one in twenty. This reduces further over the next ten years to about a one-in-fifty chance of failure in any one year.

The longer someone takes a particular HIV therapy, the less likely it is to stop working. Almost everyone who goes on to a second or third regimen reduces their viral load to an undetectable level.

If someone’s treatment does not result in their viral load becoming undetectable, this may be because they are having problems taking their treatment as prescribed. Sometimes missing a dose of medication is unlikely to cause viral load to become detectable, but frequently missing doses may lead to a detectable viral load and should be avoided.

If I’m undetectable, am I cured?

Having an undetectable viral load is not the same as being cured of HIV. Even if HIV is undetectable in body fluids, it is still present inside some cells. It may not be doing any harm there, but it would become detectable again in your blood and would start affecting your health if you stopped taking treatment.

What does U=U mean for me if I am living with HIV?

'Undetectable equals Untransmittable' has been a life-changing finding for many people living with HIV. It means that if you are on effective treatment with an undetectable viral load, you do not have to worry about passing on HIV through sex, even if you do not use a condom. This has helped many people living with HIV have more fulfilling sex lives and less anxiety around sex.

It may take some time for an HIV-negative partner to accept the U=U message and to rely on it as the sole method of preventing HIV. Some HIV-negative people may reject the message or not believe it. It may be helpful to direct your partner to information resources that explain the accuracy and significance of U=U. You could show them this page, for example. Another option could be for your partner to hear about U=U from a healthcare worker or another reliable and trusted source.

Despite sharing this information, some people may still not accept that U=U. In this kind of situation, it is important to find a balance between providing your partners with information and taking care of yourself. Many people find it difficult to talk about sex, even with the person who is closest to them. If this is the case, you might want to discuss your concerns with someone at your HIV clinic, sexual health clinic or a support organisation. This can help you clarify your thoughts and what you’d like to say.

What does U=U mean for me if my partner is living with HIV?

If you are HIV negative and have a partner who is living with HIV, who is on treatment, and who has an undetectable viral load, they will not pass HIV on to you – even if you don’t use condoms. This helps many people have more fulfilling sex lives and less anxiety around sex.

However, it might take you a while to feel confident. Many people don’t know about U=U and it can be a complicated idea to understand. You can always speak to your doctor or a sexual health clinic if you have any concerns. There is more information about the research that was used to prove U=U at the end of this page which might help you.

It’s also important to remember that if you have other sexual partners, you could still contract HIV outside of your relationship, possibly from someone who does not know that they have HIV. Remember that HIV-negative people’s status is only certain up to the last time they took an HIV test. In casual situations especially, this may mean taking somebody’s word for it or guessing their HIV status. Prevention methods such as condoms or PrEP may still be important for you.

What about sexually transmitted infections?

Regular sexual health check-ups are recommended for everyone, regardless of their HIV status. An undetectable viral load only prevents HIV transmission from an HIV-positive partner to an HIV-negative partner. It doesn’t stop the transmission of any other sexually transmitted infections (STIs) from either partner. Using condoms will help prevent STIs.

Find out more about U=U in our video we made with animation studio Animade.

Having an STI could lead to an increase in viral load for people with HIV who are not taking treatment. For example, syphilis can double viral load. With good adherence to HIV treatment, catching an STI will not raise a viral load from ‘undetectable’ to ‘detectable’. This means that effective treatment prevents sexual transmission of HIV even if there are other STIs present.

However, if you are not living with HIV, having an STI (especially syphilis) could make HIV transmission more likely if you have sex with someone living with HIV who isn’t taking effective treatment.

The scientific evidence

There have been five large studies that have looked at viral load and HIV transmission since the year 2000. They provide evidence that Undetectable equals Untransmittable (U=U).

Rakai study (2000)

The first large study indicating that people with low viral loads are not infectious came from analysis of 415 heterosexual couples in Uganda. This found that no HIV-positive partner with a viral load below 1500 copies/ml transmitted HIV.

HPTN 052 trial (2011)

This study concluded that HIV treatment reduced the risk of passing on HIV to a regular heterosexual partner by 96%. The only reason the risk was not reduced 100% is that one person in the trial did acquire HIV. However, this happened within a few days of their partner starting treatment. Over the course of the four-year study, not a single person with an undetectable viral load (below 200 copies/ml) passed HIV on to their partner.

PARTNER 1 (2016) and PARTNER 2 (2018) studies

These two studies jointly recruited 972 gay couples and 516 heterosexual couples in which one partner had HIV and the other did not. Over the course of the study, the gay couples had 77,000 acts of condomless penetrative sex and the heterosexual couples, 36,000 acts. The PARTNER studies did not find a single HIV transmission from an HIV-positive partner who had an undetectable viral load (below 200 copies/ml).

Opposites Attract (2017)

This study of 343 gay male couples found no transmissions from partners with an undetectable viral load in 17,000 acts of condomless anal sex.

Some of the HIV-negative partners in the studies mentioned here did acquire HIV. But, using genetic testing, the researchers were able to show that all these infections came from other people and not their main partners.

This means the risk of transmission by a partner with an undetectable viral load is statistically equivalent to zero.

World Health Organization review (2023)

The global health body also reviewed the scientific evidence to check for cases of HIV transmission with a suppressed viral load (below 1000 copies/ml).

In eight studies involving 4773 couples, there were only two possible cases of HIV transmission with a viral load below 1000 copies/ml. In these two cases, the last recorded viral loads were 617 copies/ml and 872 copies/ml, respectively. However, in both cases the viral load tests were taken more than 50 days before transmission occurred. This means we don’t know for sure what their viral load was when HIV was passed on.

This review is the evidence for there being ‘almost zero’ risk of HIV transmission during sex with a ‘suppressed’ viral load (below 1000 copies/ml).

U=U and conceiving a child

This page is mostly about HIV transmission from sex. However, knowing that 'Undetectable equals Untransmittable' is useful for people wishing to have a child. It means that couples in which one person has undetectable HIV and the other is HIV negative can have unprotected sex in order to conceive. Maintaining an undetectable viral load during pregnancy reduces the risk of HIV being passed on to the baby to just one in a thousand (0.01%).

If a baby is breastfed by someone living with HIV, then maintaining an undetectable viral load greatly reduces the risk of passing HIV on to the baby through the breast milk. However, it does not completely get rid of the risk. In the UK and other countries where clean water and sterilising equipment are available, bottle feeding with formula milk is safest.

We have more information about pregnancy and breastfeeding on another page.

Does U=U apply to people who inject drugs?

If you use injection drugs and share needles or other equipment, taking HIV treatment and having an undetectable viral load greatly reduces the risk of passing HIV on, but we don’t know by how much.

U=U and the law

Depending on where you live, laws about HIV may not have caught up with the science. In some countries, if you are living with HIV and having condomless sex, then not disclosing your HIV status is a criminal offence, regardless of your viral load. For information on specific countries, visit our page on criminalisation laws around the world.

Next review date