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HIV treatment and an undetectable viral load to prevent HIV transmission


It is now well known that the use of HIV treatment not only improves the health of people living with HIV, but is also a highly effective strategy to prevent HIV transmission. This is because HIV treatment can reduce the amount of virus (viral load) in the blood and other bodily fluids (such as semen and vaginal and rectal fluids) to undetectable levels. To become and remain undetectable, 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.

When a person with HIV reaches and maintains an undetectable viral load their risk of passing HIV to sex partners is negligible (meaning insignificant, or so small as to not be worth considering). Evidence shows that HIV-positive people who are on treatment, engaged in care, and have an ongoing undetectable viral load do not transmit HIV to their sexual partners. For people with HIV who inject drugs, the risk of transmitting HIV is also considerably reduced if they are on treatment and maintain an undetectable viral load, but there is currently not enough evidence to conclude that the risk is negligible.

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 viral load in the blood and bodily fluids decreases. Research tells us that as the viral load decreases, so does the risk of HIV transmission. When the viral load becomes and stays undetectable with successful treatment, the risk for sexual transmission of HIV is negligible.

ART usually consists of a combination of at least three antiretroviral drugs taken daily. Newer HIV treatments are safer, simpler and more effective than when ART was first introduced. The power of ART today is so profound that many people who start effective treatment soon after becoming HIV positive will have a near normal life span.

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. Viral load tests used in Canada cannot detect HIV in the blood if there are less than 40 to 50 copies of the virus per ml. However, the virus is still present at very low amounts in the body when the viral load is undetectable.

What is involved in the consistent and correct use of ART and an undetectable viral load for HIV prevention?

The consistent and correct use of ART to maintain an undetectable viral load includes:

  • high adherence to ART medications, to achieve and maintain an undetectable viral load
  • regular medical appointments to monitor viral load and receive adherence support, if needed

Regular testing and treatment for sexually transmitted infections (STIs) is also important since this strategy does not protect against STIs.

A person on ART needs to work with their doctor to determine an appropriate schedule for medical check-ups and viral load monitoring.

What is important for this approach to work?

After starting treatment the viral load needs to become and remain undetectable for this approach to provide protection. Research shows that HIV transmissions can happen when a person taking ART does not have an undetectable viral load.

When a person first begins treatment, it usually takes three to six months for the viral load to become undetectable. Most people will eventually have an undetectable viral load if they have a drug combination that is effective against their strain of HIV and take it as prescribed by their doctor .

The viral load should remain undetectable for at least six months before depending on this approach as an effective HIV prevention strategy. A person must continue to have high adherence to treatment to maintain an undetectable viral load over time. The only way to know if the viral load remains undetectable over the long term is to have regular viral load tests.

However, not everyone’s viral load becomes and remains undetectable on treatment. This most commonly happens when someone has low adherence to medications, but it can also occur due to drug resistance or drug toxicity. 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 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 doctor.

How well does the use of ART to maintain an undetectable viral load prevent the sexual transmission of HIV?

Research conducted in serodiscordant couples (where one partner is HIV positive and the other is HIV negative) shows that, when used consistently and correctly, the use of ART to maintain an undetectable viral load is a highly effective strategy to prevent the sexual transmission of HIV for both heterosexual and same-sex male couples. Evidence from this research shows that when people on ART are engaged in care and maintain an undetectable viral load, they do not transmit HIV through sex.

The first study to show that ART has a major prevention benefit was the randomized controlled trial known as HPTN 052. Interim results from this study showed that taking ART reduced the risk of HIV transmission by 96% among heterosexual serodiscordant couples having mostly vaginal sex. In the final analysis, which included 1,763 couples (half of whom were followed for over five and a half years), no HIV transmissions occurred between couples in the study when the HIV-positive partner was on ART and had an undetectable viral load. In total, eight transmissions occurred between these couples while the HIV-positive partner was on ART; however, in all eight cases the viral load was detectable, despite being on ART. Four transmissions occurred in the first three months after the HIV-positive partner started treatment, before the viral load was undetectable. The other four happened when treatment failed to maintain the viral load at undetectable levels. Couples in this study reported high rates of condom use, which may have partially contributed to the low number of HIV infections during the study. Although there were few transmissions between couples enrolled in the study, 26 people acquired HIV infection from a sex partner outside of the primary relationship, showing that in a serodiscordant couple in which the HIV-positive partner is on ART with an undetectable viral load, the main risk of HIV transmission comes from outside the relationship.

An observational study known as PARTNER followed 548 heterosexual and 340 gay male couples who engaged in a large number of unprotected sex acts when the HIV-positive partner’s viral load was undetectable. The couples did not use condoms, pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP). No HIV transmissions occurred, despite 36,000 unprotected sex acts among heterosexual couples and 22,000 among gay male couples. By the end of the study, 11 of the HIV-negative partners became HIV positive; however, all 11 acquired HIV from a sex partner outside of the relationship, and not from the HIV-positive partner with whom they enrolled in the study.

A preliminary analysis of another study called Opposites Attract also found no HIV transmissions among serodiscordant gay male couples when the viral load was undetectable despite over 5,000 condomless anal sex acts.

All participants in these studies were engaged in regular healthcare appointments to check viral load, test for STIs, and receive adherence and prevention counselling. They were also treated for STIs when needed. These comprehensive supports are an important part of regular follow-up care while on ART.

The results of these (and earlier) studies provide a strong body of evidence showing that people living with HIV who are adherent to ART and engaged in regular healthcare, with a sustained undetectable viral load, have a negligible risk of sexually transmitting HIV. The PARTNER and Opposites Attract studies show that this is true even when condoms are not used. Both of these studies are continuing to follow gay male serodiscordant partners to gather more data on sex when no condoms, PrEP or PEP are used.

There is one case study reported in the literature where sexual HIV transmission is suspected to have occurred in a couple where the HIV-positive partner likely had an undetectable viral load at the time of transmission. This is an exceptional suspected case within a large body of evidence, and did not occur within the context of any of the large trials.



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