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HIV treatment and an undetectable viral load to prevent HIV transmission

Summary

The use of HIV treatment not only improves the health of people living with HIV but also is a highly effective strategy to prevent HIV transmission. This is because HIV treatment can reduce the amount of virus (known as the viral load) in the blood and other bodily fluids (such as semen and vaginal and rectal fluids) to undetectable levels. To achieve and maintain an undetectable viral load, people living with HIV need to take their HIV treatment as prescribed. In addition to taking HIV medications, regular medical visits are important to monitor viral load to make sure it stays undetectable and to receive other medical support.

Evidence shows that people living with HIV who are on treatment, engaged in care and have an ongoing undetectable viral load:

  • do not transmit HIV to their sexual partners;
  • do not transmit HIV to their baby during pregnancy and delivery (if they maintain an undetectable viral load throughout pregnancy and childbirth);
  • have a greatly reduced chance of transmitting HIV through breastfeeding (also known as chestfeeding); however, breastfeeding is not recommended in Canada (exclusive formula feeding is recommended); and
  • have a reduced chance of transmitting HIV to people with whom they share injection drug use equipment; however, there is not enough evidence to conclude that there is no risk. It is recommended that people use new needles and all other equipment every time they inject drugs, regardless of their HIV status or viral load, to prevent HIV as well as other harms.

How does HIV treatment and an undetectable viral load work to prevent HIV transmission?

HIV treatment, also called antiretroviral therapy (ART), works by controlling the replication of HIV in the body — that is, it reduces HIV’s ability to make copies of itself. When HIV replication is controlled, the amount of virus (also known as the viral load) in the blood and other bodily fluids decreases. Research tells us that as the amount of virus in the body decreases, so does the risk of HIV transmission. When successful treatment lowers the viral load to undetectable levels, this can reduce or even eliminate the risk of HIV transmission.

HIV treatment usually consists of a single pill once a day; however, long-acting injectable formulations are also now available. Newer HIV treatments are safer, simpler and more effective than when treatment was first introduced. The power of treatment today is so profound that many people who start effective treatment soon after becoming HIV positive will have a normal lifespan.

For most people the virus becomes so well controlled that within three to six months of starting treatment the amount of virus in their blood becomes undetectable by routinely used tests. Most viral load tests used in Canada cannot detect HIV in the blood if there are fewer than 40 to 50 copies/ml of the virus but some newer tests can detect as few as 20 copies/ml. The virus is still present in very low amounts in the body when the viral load is undetectable.

What is important for this approach to work? 

For HIV treatment to prevent HIV transmission, a person’s viral load needs to become and remain undetectable after they start treatment. Achieving this requires taking HIV medications consistently and correctly, as well as attending regular medical appointments to monitor viral load and get adherence support if needed.

When a person begins treatment, it usually takes three to six months for their viral load to become undetectable. Most people will reach an undetectable viral load if they are on effective HIV treatment and take their medications as prescribed.

It is recommended that a person maintain an undetectable viral load for at least six months before relying on it as their main HIV prevention strategy. Regular viral load tests are the only way to confirm that the virus remains undetectable over time. A person should work with their healthcare provider to set up an appropriate schedule for checkups and viral load monitoring.

However, not everyone’s viral load becomes and remains undetectable on treatment. The most common reason why a person’s viral load stays detectable is low adherence to their medications, but drug resistance may also be the issue. When treatment fails, a person won’t know that their viral load is detectable until they get another viral load test. Depending on the reason the treatment failed, a person may require a change in treatment, or they may benefit from adherence counselling, to bring their viral load back down to undetectable levels. The best options for moving forward should be discussed with a healthcare provider.

What is the evidence on the effectiveness of HIV treatment and an undetectable viral load to prevent sexual HIV transmission?

Research conducted in serodiscordant couples (where one partner is HIV positive and the other is HIV negative) shows that consistent and correct use of HIV treatment to maintain an undetectable viral load is a highly effective strategy to prevent sexual HIV transmission for both heterosexual and same-sex male couples. The following studies in serodiscordant couples have provided strong evidence to show that when people are on successful treatment and engaged in care there is no risk of transmitting HIV through sex:

  • The HPTN 052 trial found no HIV transmissions between heterosexual couples where the HIV-positive partner was on treatment and maintaining an undetectable viral load (defined as <400 copies/ml in this study). All observed transmissions occurred when the HIV-positive partner’s viral load was detectable—either soon after starting treatment or when treatment failed. Additional observed infections in the study came from outside HIV-positive partners who were not part of the study.
  • The PARTNER/PARTNER2 study found that treatment and an undetectable viral load (defined as <200 copies/ml in this study) prevents sexual HIV transmission in both heterosexual and same-sex male couples in the absence of other forms of HIV prevention (condoms, pre-exposure prophylaxis [PrEP] or post-exposure prophylaxis [PEP]). Across more than 100,000 instances of condomless sex without PrEP or PEP, there were no transmissions between couples in the study when the HIV-positive partner was on treatment and had an undetectable viral load. Sixteen new infections in the study were acquired from an HIV-positive sex partner outside the primary relationship.
  • The Opposites Attract study also found no HIV transmissions between same-sex male couples when the HIV-positive partner was on treatment and maintained an undetectable viral load (<200 copies/ml in this study) despite approximately 16,800 condomless anal sex acts. In this study, three of the HIV-negative partners got HIV from an HIV-positive partner outside of the relationship.

The PARTNER/PARTNER2 and Opposites Attract studies defined “undetectable” as <200 copies/ml with zero observed transmissions below this level. This shows that even in the presence of temporary viral load blips there is no increased risk for HIV transmission. While most participants did have a viral load below 50 copies/ml, the higher cut-off captured minor fluctuations in viral load and allowed for comparison across studies. In Canada, maintaining a viral load of less than 50 copies/ml remains the goal to prevent drug resistance and viral rebound that can lead to treatment failure. 

The PARTNER/PARTNER2 and Opposites Attract studies also showed that STIs do not increase HIV transmission risk when the HIV-positive partner is on treatment and has an undetectable viral load. Across these studies, many STIs were recorded and thousands of condomless sex acts occurred during STI episodes, but no HIV transmissions were observed.

The results of these (and earlier) studies provide a strong body of evidence showing that people living with HIV who adhere to their treatment and engage in regular healthcare, with a sustained undetectable viral load, do not transmit HIV sexually. This is true even when condoms are not used, and in the presence of other STIs.

What is U=U?

U=U stands for “Undetectable equals Untransmittable”. This phrase was created to get the word out about the benefits of HIV treatment for preventing HIV during sex. It is based on solid scientific evidence that an individual living with HIV who is taking HIV treatment, and who has maintained an undetectable viral load cannot transmit HIV to their sexual partners, with or without the use of condoms.  

What is the evidence on the effectiveness of HIV treatment and an undetectable viral load to prevent HIV transmission to a baby during pregnancy and birth? 

Without treatment, there is a 15% to 30% chance that a baby born to a person living with HIV will get HIV during pregnancy or delivery. Taking HIV treatment is the most effective way to reduce transmission to the baby. In fact, research suggests that if a pregnant person starts HIV treatment before conception and maintains an undetectable viral load throughout pregnancy and delivery, they do not transmit HIV to their baby. A short course of HIV medications is also given to the infant to prevent HIV transmission. If a person is not on treatment when they become pregnant, starting HIV treatment as soon as possible dramatically lowers the chance of transmitting HIV to their baby.  

For the last three decades, a growing body of evidence has shown that babies are much less likely to be born with HIV if the pregnant parent is on treatment. A study of the French Perinatal Cohort, conducted between 2000 and 2017, is the largest study to show the impact of treatment on preventing HIV transmission to a newborn. This study found that no HIV transmissions occurred among 5,482 infants born to cisgender women who were on treatment before they conceived and throughout their pregnancy, who had an undetectable viral load (defined as less than 50 copies/ml) at delivery and who did not breastfeed.

The French study also showed that starting HIV treatment as soon as possible in pregnancy dramatically lowers the chance of transmission. In the study, there was a 0.52% chance of passing HIV to a baby if treatment was started in the first trimester, a 0.75% chance if treatment was started in the second trimester and a 1.67% chance if treatment was started in the third trimester. Viral load was not taken into account in this analysis.

It is important that people who are pregnant or considering becoming pregnant get tested for HIV. People who test positive should begin HIV treatment as soon as possible to reduce or eliminate the risk of passing HIV to their babies. Likewise, people living with HIV who wish to become pregnant should consult with an HIV specialist as soon as possible, preferably before conception, to determine a suitable treatment regimen for pregnancy.

What is the evidence on the effectiveness of HIV treatment and an undetectable viral load to prevent HIV transmission to a baby during breastfeeding?

Without HIV treatment, the risk for HIV transmission through breastfeeding is estimated to be roughly 15%. The rates of HIV transmission through breastfeeding for people who are taking HIV treatment are much lower. A systematic review of HIV transmission in breastfed infants of cisgender women on treatment found that the risk of transmission was 1% after six months of breastfeeding, rising to almost 3% after one year. However, in these studies, the women were on treatment for varying amounts of time and did not continue treatment beyond six months after giving birth. The systematic review did not account for adherence or for viral load, which means that even though the women were taking HIV treatment we do not know how many of them had a detectable viral load at the time of transmission.

Only a few studies have measured the risk of transmission through breastfeeding when the mother was on treatment and had an undetectable viral load. The largest study to do so is called PROMISE and was done in several African countries and India. Among the 1,220 infants whose mother was taking treatment, seven acquired HIV through breastfeeding, an infection rate of 0.57%. In five out of the seven cases, the mother had a detectable viral load at their most recent viral load test before the baby tested positive. In the other two cases, the mother had an undetectable viral load at their most recent viral load test. This means that some infants acquired HIV despite the mother being on treatment and having an undetectable viral load near the time of transmission.

Three smaller studies have also looked at HIV transmission through breastfeeding in the context of treatment and an undetectable viral load. Two of those studies found no transmissions when the mother was taking treatment and had an undetectable viral load throughout the time they were breastfeeding. In the third study, two infants acquired HIV although their mothers had an undetectable viral load at their last test. Taken together, these studies suggest that the risk of transmission when the parent is on treatment and has an undetectable viral load is very low but not zero.

Canadian guidelines continue to recommend that HIV-positive parents exclusively feed their babies formula to eliminate the possibility of transmission. However, guidelines also recommend supporting people with HIV who wish to breastfeed and helping them do so as safely as possible. This includes offering unbiased information on the risk of HIV transmission through breastfeeding, providing increased viral load monitoring and adherence support, and providing prophylactic treatment for infants born to people living with HIV. Several case studies have documented women with HIV in North America who were supported to breastfeed by their medical team. None of these babies acquired HIV.

What is the evidence on the effectiveness of HIV treatment and an undetectable viral load to prevent HIV transmission through injection drug use?

The limited available research suggests that being on HIV treatment and maintaining an undetectable viral load is effective at helping to prevent HIV transmission among people who inject drugs; however, people who use drugs can get HIV through sex and through sharing injection drug use equipment. While we know that maintaining an undetectable viral load will prevent HIV transmission through sex, we don’t know how much it reduces the chance of passing HIV through shared injection drug use equipment. The best way to prevent passing HIV through injection drug use is to use new needles and other equipment every time. People who inject drugs need access to enough new equipment to be able to do this consistently and to avoid having to share with others.

The three major studies looking at sexual HIV transmission (HPTN 052, PARTNER and Opposites Attract) did not systematically recruit people who inject drugs, they did not ask whether participants were sharing injection equipment and they did not provide any analysis related to participants who reported using drugs.

Two ecological studies from Vancouver and Baltimore reported on reductions in new HIV infections over time and found an association with a reduction in the community viral load of people who inject drugs. Although it is likely that increased uptake of HIV treatment is partly responsible for the observed decline in the number of new infections, the success of intensive harm reduction interventions during this period also contributed towards the observed decline in HIV incidence. A cohort study in India among 14,481 people who inject drugs and 12,022 men who have sex with men found a clear correlation between estimated HIV incidence and both community-level treatment coverage and viral suppression. This study found significant correlations at the community level, but since it was not designed to look at individual risk of transmission, no estimate of effectiveness was available.

CATIE resources

The Power of Undetectable: How HIV Treatment Prevents Transmission - booklet

HIV Treatment to Prevent HIV - video

Can’t Pass It On – awareness campaign

CATIE Statement on the use of antiretroviral treatment (ART) to maintain an undetectable viral load as a highly effective strategy to prevent perinatal transmission of HIV

U=U: A guide for service providers

Guidelines, position papers and consensus statements

Expert consensus statement on the science of HIV in the context of criminal law (2018) – Journal of the International AIDS Society

Position ministérielle - L'effet du traitement des personnes vivant avec le VIH sur le risque de transmission sexuelle de l'infection – Ministère de la Santé et des Services sociaux du Québec

Consolidated guidelines on HIV prevention, diagnosis, treatment and care for key populations – World Health Organization (WHO)

Human immunodeficiency virus (HIV) Sexual Transmission Risk with Bacterial Sexually Transmitted Infection (STI) Co-infection – Public Health Ontario

Canadian HIV Pregnancy Planning Guidelines

Canadian Pediatric & Perinatal HIV/AIDS Research Group consensus recommendations for infant feeding in the HIV context  

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Author(s): Arkell C, Harrigan M.

Updated: 2026