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Why, for HIV, Undetectable = Untransmittable

April 14, 2017

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In recent months, an increasing number of national and international scientists, advocates and organizations have issued statements about undetectable viral loads, HIV treatment and HIV transmission.

The most prominent comes from Prevention Access Campaign, the U.S.-based activist group that coined the "Undetectable = Untransmittable" slogan. They spell out what this means:

People living with HIV on [antiretroviral treatment] with an undetectable viral load in their blood have a negligible risk of sexual transmission of HIV.

They clarify that "negligible" means "so small or unimportant as to be not worth considering; insignificant."

Their statement continues:

Depending on the drugs employed it may take as long as six months for the viral load to become undetectable. Continued and reliable HIV suppression requires selection of appropriate agents and excellent adherence to treatment. HIV viral suppression should be monitored to assure both personal health and public health benefits.

Prevention Access Campaign's statement has been endorsed by some of the leading scientists working on this issue. They include Myron Cohen, M.D., of the University of North Carolina (the HPTN 052 study); Jens Lundgren, M.D., of the University of Copenhagen (the PARTNER study); and Pietro Vernazza, M.D., of St. Gallen Hospital (the "Swiss Statement").

The statement has also been endorsed by over 150 organizations. In the United States, endorsers include AIDS United, GMHC, Housing Works, Human Rights Campaign, Latino Commission on AIDS, National Black Justice Coalition, the Positive Women's Network - USA, the San Francisco AIDS Foundation, Southern AIDS Coalition and us -- The International AIDS Society and a large number of organizations in Europe, Australia, Canada and South Africa have also given their support.

Similar language has been used by some of the world's most prominent HIV scientists. In a statement on World AIDS Day 2016, Anthony S. Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases (NIAID) and Carl W. Dieffenbach, Ph.D., director, Division of AIDS, NIAID, said:

[S]tudies have proven that when an individual living with HIV is on antiretroviral therapy and the virus is durably suppressed, the risk that he or she will sexually transmit the virus is negligible.


How Can Treatment Have This Effect?

With all viral infections, the potential for passing on the infection depends on the quantity of virus in body fluids. When there's a large quantity of virus, transmission is more likely. Having a smaller (or non-existent) quantity of virus makes transmission less likely (or impossible).

HIV treatment works by stopping HIV from replicating in cells, which dramatically reduces the quantity of HIV in the body. Although treatment is unable to cure people of HIV (the virus will still be hiding out in some places, such as the gut), treatment can virtually eliminate HIV from some body fluids. Crucially, these include semen, vaginal fluids and blood.

The impact of treatment can be measured with a blood test, called a viral load, which measures the quantity of virus in blood. When the result is described as "undetectable," this means that, in the sample of blood that was analyzed, the test was unable to detect any HIV or found only a tiny trace.

After a few months of taking HIV treatment, most people with HIV have an undetectable viral load. It should stay that way so long as the pills are taken every day, without interruptions.

What Evidence Do We Have for This?

Pietro Vernazza, M.D., is one of the Swiss physicians who first delivered the "Undetectable = Untransmittable" message in the Swiss Statement of 2008.

He recently said that the evidence he finds most convincing is the simple absence of people who have acquired HIV sexually from a partner who was taking treatment and had a fully suppressed viral load.

He called this the "non-observation argument." Documented case reports of HIV transmission in those circumstances are absent from the world's medical journals. This absence was striking a decade ago, but it is even more so in the years since HIV treatment and transmission became a hot topic for scientists.

"[O]ne might expect an even higher degree of scrutiny amongst physicians to investigate and publish suspected cases compared with the years before 2008," Vernazza wrote. "Therefore, a further lack of documented cases should be even more convincing than it was in 2008."

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