A recent fact sheet published by the Centers for Disease Control and Prevention (CDC) examines whether HIV-infected individuals with undetectable viral load, or amount of virus in the blood, are less infectious and are thus less likely to spread infection through sexual transmission.

Antiretroviral treatment (ART) includes medications that are able to decrease the replication of virus, thereby decreasing the amount of virus in the body. HIV-infected individuals undergoing ART can often suppress viral load to undetectable levels.

Studies in Uganda, Spain and San Francisco of heterosexual populations, or of men who have sex with men, found evidence that ART may decrease the likelihood of sexual transmission from infected individuals to their partners. In areas with greater ART availability, individuals had lower viral loads and there was a lower sexual transmission rate of the virus in the population.

The efficacy of ART in decreasing transmission of HIV within populations may depend on testing and early identification of infected individuals, counseling to encourage healthy sexual behaviors, regular clinical monitoring of effects of ART, and greater accessibility to treatment geographically and financially.

For couples in which one member is HIV-infected and has undetectable viral loads due to ART, the risk of sexually transmitting the virus is greatly decreased. However, the risk is not always eliminated. Additionally, the chances of transmission are expected to increase with greater exposure.

For couples in which both partners are HIV-infected, the relevance of superinfection has yet to be determined. Superinfection is infection by a second strain of HIV after previously being infected. For now, it is recommended that individuals be aware of the risk and use additional preventive methods, such as condoms, to decrease chances of transmission.

The possibility of HIV sexual transmission in individuals undergoing ART could be affected by several factors:

1. ART effectiveness is measured by monitoring only the blood plasma viral load periodically. Thus, virus may exist and persist in seminal fluid cells of men who have undetectable viral load under ART.
2. Individuals undergoing ART with undetectable viral load may experience periodic temporary increases in plasma viral load, generally small increases lasting a few weeks. These transient increases may relate to increases in genital fluid viral load and greater sexual transmission risk.
3. Undetectable plasma viral load may not always predict undetectable seminal fluid viral load, thus an individual may spread virus in higher levels through genital fluid.
4. Sexually transmitted infections (STIs) such as gonorrhea and chlamydia may increase genital viral load.
5. The type of ART medication may also cause variation in genital fluid viral load due to differences in the way the medications enter genital fluid. For example, nucleoside/tide reverse transcriptase inhibitors penetrate to a greater extent in genital secretions than protease inhibitors.
6. Through mutations that occur during replication, HIV may become resistant to ART medications.
7. Medications that do enter the genital fluid may lead to the development of resistance to ART in the genital fluid.

ART is an effective method in decreasing viral load in HIV infected individuals. However, infected individuals should recognize that undergoing ART and having low viral loads may not directly lead to a decrease in sexual transmission of the virus. Thus, it is recommended that the individuals examine all the risks and use additional preventive methods.

For more information, please see CDC’s fact sheet.