Undetectable Viral Load: As Safe as It Sounds?

Published: June 13, 2013

The past four weeks have seen the 30th anniversaries of two milestone events. In mid-May 1983, the booklet How to Have Sex in an Epidemic was published, and two weeks later the Denver Principles were drafted. Decades later, gay men continue to explore possibilities around HIV prevention; the epidemic also continues. Dr. Joseph Sonnabend, a physician and AIDS researcher and co-author of How to Have Sex in an Epidemic, shares his thoughts on the pros and cons of one of the latest approaches.

Mark Adnum: In terms of the risk of transmission of HIV, what does it mean if someone is "undetectable"?

Dr. Joseph Sonnabend: In the context of transmission risk, "undetectable" generally refers to a failure to detect HIV in the blood stream. "Undetectable" does not mean it’s not there, but that it can’t be found by a particular test.

There is a relationship between viral load and transmission probability, which falls as viral loads decrease. This is evident from studies on populations. In principle this should also be the case on an individual level, but here there are some practical considerations.

For example, the significance regarding transmission of a single undetectable viral load three months ago, or even yesterday, is uncertain. Since we can’t know the viral load at the time of sex, we would need additional information; being undetectable without qualification may not be enough information to make a choice about unprotected sex. In 2008 Swiss researchers suggested a set of circumstances under which an individual with an undetectable viral load might be considered to be incapable of transmission. According to them, HIV-infected individuals on effective suppressive antiviral treatment who had undetectable viral loads for at least six months and who had no sexually transmitted infection could be considered to be non-infectious. This view was certainly not shared by everyone.

Maybe much of the problem is in the wording: An assurance of zero risk is just not attainable. I do essentially agree with the idea behind the Swiss statement — at least to the extent that it’s possible to define circumstances where the probability of transmission is extremely low — even if it’s not possible to precisely quantify, and allow individuals to make an informed decision.

Given that the receptive partner is at greater risk, their best protection is of course to refuse sex with a partner who will not use a condom. Among men who have sex with men, there surely can only be very limited situations when the receptive partner would choose the more dangerous course. We should also make clear what we know about risk, and provide more information than simply relative risk reduction numbers.

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