HIV viral load is the amount of HIV (or number of virus) in the bodily fluids of someone living with HIV. It is measured in the blood as part of routine clinical care. A higher viral load is associated with a higher risk of HIV transmission. Research shows that successful HIV treatment can reduce the viral load to "undetectable" levels and this can reduce the risk of HIV transmission. However, HIV transmission may be possible when the viral load is undetectable because there is still virus present in the blood and other bodily fluids. The risk of HIV transmission when taking antiretroviral treatment may increase if sexually transmitted infections (STIs) are present, doses of medications are missed, or drug resistance develops. This risk may also be higher for anal sex than for vaginal sex.
HIV viral load is the number of copies of HIV in the bodily fluids of someone living with HIV. It is measured as the number of copies of the virus in one millilitre of fluid (copies/ml). Viral load is measured in the blood and is used to monitor the progression of HIV infection and the success of HIV treatment. It is not commonly measured in other bodily fluids, such as semen, vaginal fluid or rectal fluid.
HIV treatment consists of a combination of at least three drugs that are normally taken daily. The goal of HIV treatment is to reduce the production (also called replication) of HIV, raise levels of CD4 T-cells, and slow disease progression. HIV treatment is also called highly active antiretroviral treatment (HAART) or antiretroviral therapy (ART).
With successful HIV treatment, the viral load can become very low or "undetectable" in the blood and other bodily fluids.
There is no such thing as a "normal" viral load. The viral load in the bodily fluids can change as a result of several factors, such as the stage of HIV infection and HIV treatment.
During the first few weeks after becoming infected with HIV, the viral load in the blood and other bodily fluids is very high. This stage of HIV infection is known as the acute infection stage and at this time the viral load can reach levels higher than 1 million copies/ml.
The acute HIV infection stage only lasts for a few weeks and then the chronic stage of HIV infection begins. During the chronic stage, the viral load begins to decrease and -- after a few months -- the viral load stabilizes at a lower level.
If HIV treatment is started, the viral load can be reduced to "undetectable" levels in the bodily fluids within a few months. However, if doses of medications are missed or HIV develops resistance to treatment, then the viral load will increase.
"Undetectable" means that the number of virus in the blood is below the limit that viral load tests can detect. Viral load tests used in Canada cannot detect HIV in the blood if there are less than 40-50 copies/ml. Therefore, an undetectable viral load means the amount of virus in the blood is too low to detect, it does not mean that there is no virus present.
Research shows that a lower amount of virus in the blood is usually associated with a lower risk of transmitting HIV to others, and a higher viral load is associated with a higher risk.
The amount of virus in the blood is usually correlated with the viral load in the semen, vaginal fluid, and rectal fluid (the fluids commonly involved in the sexual transmission of HIV). This means that when the viral load in the blood decreases, it generally also decreases in the other fluids.
However, the viral load in the different bodily fluids is never exactly the same. For example, the viral load in the semen, vaginal fluid or rectal fluid can sometimes be higher than the viral load in the blood.
Successful antiretroviral treatment can lower the viral load in the blood and other bodily fluids to undetectable levels and this can reduce the risk of sexual HIV transmission.
A randomized controlled study known as HPTN 052 found that HIV treatment reduced the risk of HIV transmission between serodiscordant heterosexual couples by 96% (equivalent to a 26-fold reduction in risk). A serodiscordant couple is where one partner is HIV-positive and the other is HIV-negative.
Couples in the HPTN 052 study were mostly heterosexual, mostly reported having vaginal sex, and were provided with regular adherence counselling, viral load tests, testing and treatment for sexually transmitted infections (STIs), and prevention counselling and free condoms. Therefore, this study demonstrated the effectiveness of treatment in reducing the risk of HIV transmission through vaginal sex when pills are taken regularly, drug resistance is monitored, and STIs are managed. Antiretroviral treatment may be much less effective than 96% when these conditions are not met.
No studies have been completed among populations who mostly have anal sex, such as some gay men or other men who have sex with men (MSM). However, a working group meeting hosted by the World Health Organization in 2011 concluded that "there is reason to believe that early initiation of ART for HIV prevention will benefit MSM, transgender women, and others who have anal intercourse, although the magnitude of the effect may be different from that observed in serodiscordant heterosexual couples." In other words, HIV treatment reduces the risk of HIV transmission for gay men and other MSM, but it may or may not be as effective as for heterosexual couples in the HPTN 052 study.
There are ongoing studies that are trying to get a better idea of how well HIV treatment can reduce the risk of HIV transmission among gay men and other MSM.
Although the risk of sexual HIV transmission is reduced when the viral load is undetectable, the risk of HIV transmission may not be eliminated.
Many people who have an undetectable viral load in the blood also have an undetectable viral load in other bodily fluids. However, undetectable does not mean that there is no virus, only that the amount of virus is below the limits that tests can detect. Therefore, HIV transmission may still be possible because there is still virus present.
Also, it is possible for people who have an undetectable viral load in the blood to sometimes have detectable (although lowered) levels of virus in their other bodily fluids. A higher level of HIV in the semen, vaginal fluid, and rectal fluid may increase the risk of transmission when the blood viral load is undetectable. However, it is unclear how often this happens and how significant it is in terms of HIV transmission. Research shows it may be more common if a person has an STI, but can also happen in the absence of STIs.
Although we know having an undetectable blood viral load can greatly reduce the risk of HIV transmission, it is unclear exactly what this risk is reduced to.
In the research conducted so far, there have been no recorded HIV transmissions among heterosexual couples where the HIV-positive partner is on treatment and their blood viral load is undetectable. However, this does not mean the risk through condomless sex is zero. All of the couples studied to date have also reported using condoms often. This makes it difficult to determine the risk of HIV transmission when no condom is used.
Although there have been no studies among gay men and other MSM, there has been one report of HIV transmission occurring between two men when the HIV-positive partner had an undetectable viral load.
Also, the risk of HIV transmission when the viral load is undetectable may not be the same for all types of sex. This risk may be higher for anal sex than for vaginal sex, particularly if the HIV-negative partner is the receptive partner (bottom) during anal sex. This is because receptive anal sex generally carries a higher baseline HIV risk than other types of sex.
There are ongoing studies following serodiscordant heterosexual and same-sex couples who are taking HIV treatment, have an undetectable viral load, and do not always use condoms. These studies will provide a better understanding of the risk of HIV transmission when the viral load is undetectable and no condom is used.
There are no simple answers on viral load, HIV treatment and their relationship to HIV transmission and prevention. However, there are key messages for those who want to use HIV treatment to reduce their risk of HIV transmission:
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