Medications used to treat HIV infection (antiretroviral drugs) help many people with HIV to lower the levels of virus in their blood (viral load) to undetectable levels. Effective treatment with these medications (antiretroviral therapy, or ART) may decrease the chance that an infected person will transmit HIV to others through sex. However, the risk of spreading infection is still not zero, which means that persons with HIV who are taking ART, or persons who are in a relationship with someone who has HIV and is taking ART, should still use proven prevention methods, such as condoms.

ART helps control HIV by preventing it from reproducing itself in the body, which results in a decreased amount of virus in the blood (viral load). ART is usually taken as a combination of three or more drugs. A person taking ART will have his or her blood monitored every few months to measure viral load. ART is thought to be working effectively when the blood viral load is undetectable—that is, when the viral load test cannot detect any virus in the blood.

Having an undetectable viral load does not mean that there is no virus in the body. Rather, it means that the amount of virus is so low that the test cannot detect it. Viral load tests are not perfect and use only a small sample of blood. If there is very little virus in the sample, the test may miss it. There may also be temporary increases in viral load between tests. In addition, the test is only able to detect virus that is in the liquid part of the blood (the plasma). HIV can still be detected inside blood cells and in other tissues in the body, including genital fluids, even when it is not detectable in the liquid part of blood (plasma). ART does not completely eliminate HIV once someone is infected.

There is some information suggesting that the chances of transmitting HIV through sex are lower when the infected partner is taking ART and has an undetectable viral load. For example, a study in Rwanda and Zambia of heterosexual couples with one infected partner noted that transmission was much less likely to occur if the infected partner was receiving ART. Another study of heterosexual couples in Uganda found that infected people who did transmit virus to their partners had higher average viral loads than those who did not. There is less information about how ART affects the chances of spreading HIV in men who have sex with men (MSM), which is the primary way HIV is transmitted in the United States. However, there are studies of ART’s effects on populations. For example, one study of MSM in San Francisco between 1994 and 1999 (the period during which effective ART first became available) noted a drop in the number of new infections. Other studies that use mathematical models suggest that if enough infected people in a population used ART, this could substantially lower the spread of HIV within that population. However, it is important to emphasize that ART can only reduce transmission within a population if risk behavior does not increase. A later study of MSM in San Francisco between 1998 and 2007 found that there was no decrease in new HIV infections, possibly because there was also no decrease in high-risk behavior during this period.

Taken together, this information suggests that the use of ART by persons with HIV may help slow the spread of HIV in a population, which is different from focusing on individuals. For this reason, ART may be a very important and powerful prevention tool—if enough people know their HIV status, are taking ART if they need it, and are able to lower their viral load to undetectable levels. However, because persons with undetectable levels of HIV still carry the virus, transmission of the virus will still occur in some couples. Therefore, it is important that individual couples use other proven prevention methods, such as condoms, in order to reduce the risk of transmitting HIV to the uninfected partner.

There are a number of factors that can increase the chance of spreading HIV through sex, even if the infected partner’s viral load is undetectable. One issue has to do with viral load in genital fluids (semen and vaginal secretions), which may be transferred during sex. HIV may be found in the genital fluids of both men and women infected with HIV. The viral load test only measures virus in blood. While ART may lower viral load in genital fluids, it may not lower it as much or as quickly as it does in blood.

Another factor is related to temporary increases in viral load, sometimes called “blips”. Blips are small increases that may happen occasionally, and may last days to weeks. They can occur in between the viral load tests done by a health care provider and there are generally no symptoms or other signs to suggest that they are occuring. As a result, even though the viral load may be undetectable when blood is tested every few months, it may not always be undetectable between tests.

Also, sexually transmitted infections (STIs) like gonorrhea and chlamydia have been shown to increase viral load in genital fluids. HIV-infected people with these infections may be more likely to transmit HIV to others, even if the blood viral load is undetectable. Because people with STIs may not have any symptoms, it may be impossible for either partner to be aware of the increased risk.

Finally, it is important to take ART correctly and consistently. Not taking all the medications, not taking a high enough dose, and not taking them on the right schedule can give the virus a chance to reproduce, and can also allow it to become resistant to medications. Missing medication for even a short period could allow the viral load to increase enough to make transmission more likely.

When both partners are infected with HIV, it is still possible for one partner to transmit his/her virus to the other. This situation, in which the one person is infected with a second strain of virus, is called “superinfection”. It is not known how often superinfection occurs and studies examining different groups of infected people have found different results. Superinfection can have negative effect on the course of HIV. There have been cases in which it caused the disease to progress more quickly, or caused treatment problems because the superinfecting strain was resistant to medications. If both partners are taking effective ART, the chance that superinfection will occur is probably decreased. However, the risk can never be zero for all of the reasons mentioned above: increased viral load in genital fluids, blips in viral load, or the presence of an undetected sexually transmitted infection. To protect against superinfection, couples should consider using proven prevention methods (such as condoms)—even if both partners have undetectable viral loads.

Effective ART that suppresses viral load to undetectable levels may be a promising tool to help slow the spread of HIV in populations. It also may help individual couples to lessen their chances of transmitting the virus from one partner to the other through sex.  However, it is important to realize that the risk of transmission is not completely eliminated. To help prevent the spread of HIV through sex, the following are important:

• Know your HIV status—get tested:  Knowing whether or not you have HIV is the first step toward keeping yourself healthy and avoiding passing infection on to others. Continue to get tested regularly if you engage in ongoing risk behavior.     
• If you are HIV infected, know about ART: See a healthcare provider and find out if you should be on ART. Even if you do not need ART at first, keep your appointments for check-ups so that you will be able to start when you do need it. Current guidelines suggest that ART be started when the CD4 cell count is between 350-500. However, it may be started when CD4 counts are greater than 500, depending upon your situation. For example, pregnant women and people with certain medical conditions should start earlier, at higher CD4 counts. ART can also be started earlier to help prevent HIV transmission to partners at risk for infection. Ask your healthcare provider about when the time is right for you.
• If you are on ART, take it correctly and consistently: ART drugs work best when the right doses are taken at the right times. Not taking them properly gives the virus a chance to multiply and sometimes become resistant to the medications. Taking ART as recommended will give you the best chance of staying healthy, and will probably help lessen the chance of infecting others.
• Whether you are infected or not, know what to do to prevent transmission of HIV: Effective ART and an undetectable viral load will probably decrease the risk of transmission, but ART alone will not prevent all new infections. For additional protection, other prevention methods—abstinence, sex only within a mutually monogamous relationship, and condoms—should be used.