Spotlight Center on HIV Prevention Today

Insight Into HIV Transmission Risk When the Viral Load Is Undetectable and No Condom Is Used

April 10, 2014

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Risk Per Act of Sex

Upper confidence limits were calculated for the risk of HIV transmission per act of condomless sex with an HIV-positive partner:

  • Receptive vaginal sex (with or without ejaculation): 0.028%
  • Insertive vaginal sex: 0.027%
  • Receptive anal sex (with or without ejaculation): 0.05%
  • Insertive anal sex: 0.033%

For example, the upper confidence limit for receptive anal sex can be interpreted as follows:

Given the number of receptive anal sex acts that occurred and the fact that no HIV infections were observed, it is extremely likely that the "true" risk is somewhere between 0% and 0.05%, and extremely unlikely that it is above 0.05%. Although it is still possible that the "true" risk is zero, or only slightly higher than zero, the researchers could not rule out the possibility that the risk is as high as 0.05%.

Despite this uncertainty, this research is still meaningful. Many of the upper confidence limits are lower than the HIV transmission risks that have been estimated for when a person's viral load is detectable. For example, the average per-act transmission risk for receptive anal sex has been estimated to be 1.4% when the viral load is detectable. However, the PARTNER study found that this risk is extremely likely to be below 0.05% when the viral load is undetectable.

Previous studies have estimated the average risk per condomless sex act when the viral load is detectable to be 0.08% for receptive vaginal sex, 0.04% for insertive vaginal sex and 0.06-0.62% for insertive anal sex. Therefore the PARTNER study strongly suggests that ART reduced the risk of HIV transmission for all types of sex.

Risk Over Time


Upper confidence limits were also calculated for a couple's 10-year risk of HIV transmission when the HIV-positive partner has an undetectable viral load:

  • Receptive vaginal sex (with or without ejaculation): 11.7%
  • Insertive vaginal sex: 11.4%
  • Receptive anal sex (with or without ejaculation): 17.9%
  • Insertive anal sex: 12.8%

For example, the upper confidence limit for receptive anal sex can be interpreted as follows:

Given that couples were having condomless sex about once a week, it is extremely likely that the 10-year risk of HIV transmission is somewhere between 0 and 17.9% for receptive anal sex. While it is still possible that the "true" risk is zero, or only slightly higher than zero, the study could not rule out the possibility that the risk is as high as 17.9%.

It is important to note that the size of the upper confidence limit reflects the effects of chance. Receptive anal sex has the highest upper confidence limits because there were fewer receptive anal sex acts during the study compared to other types of sex. The lower number of sex acts means chance may have played a greater role.


The preliminary results from the PARTNER study provide important and encouraging new insight into the risk of transmitting HIV sexually when a person's viral load is undetectable and no condom is used. These results can help serodiscordant couples assess their HIV risk and make informed decisions.

The investigators of the PARTNER study concluded that the overall risk of HIV transmission through condomless sex for couples in stable serodiscordant relationships (when the HIV-positive partner is on ART, receives regular HIV care, and has an undetectable blood viral load) is "extremely low, but uncertainty over the risk remains, particularly over receptive anal sex. Additional follow-up in MSM (men who have sex with men) is essential to provide more precise estimates for transmission risk given the current assumptions of safety in some communities."

As the PARTNER study continues to follow couples who continue to have sex, the upper confidence limits will become "tighter" and move closer to zero -- assuming that no HIV transmissions occur. This will allow the investigators to more confidently conclude that the risk of HIV transmission is "extremely low" for all types of vaginal and anal sex. The PARTNER study will continue to follow heterosexual couples until April 2014. Same-sex male couples will be followed until 2017 and the investigators plan to enroll 450 more gay male couples.


Official Q&A for PARTNER Studies

HPTN 052: The trial that changed everything -- TreatmentUpdate

Undetectable viral load and HIV transmission risk: results of a systematic review -- CATIE News

Treatment and viral load: what do we know about their effect on HIV transmission? -- Prevention in Focus.

Putting a number on it: the risk from an exposure to HIV -- Prevention in Focus


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This article was provided by Canadian AIDS Treatment Information Exchange. It is a part of the publication CATIE News. Visit CATIE's Web site to find out more about their activities, publications and services.

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