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Why, for HIV, Undetectable = Untransmittable

April 14, 2017

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Some people might wonder whether such cases exist but simply haven't been noticed by doctors. To make sure that wasn't the case, scientists set up the PARTNER study. The aim was to observe and carefully measure the real life risk of HIV transmission in couples that weren't using condoms.

The researchers specifically sought out couples in which one partner was taking HIV treatment and the other partner did not have HIV. Moreover, they only enrolled couples who were already having sex without condoms.

They collected data on 888 couples who had sex without condoms (or pre-exposure prophylaxis [PrEP]) when viral load was undetectable (below 200 copies/mL). There were 58,000 unprotected penetrative sex acts, around one-third in gay couples and two-thirds in heterosexual couples.

And how many people acquired HIV from their partners?

Zero. Not one HIV-negative partner contracted HIV from a positive partner. No transmissions took place within these couples.

What's more, all of the factors that someone might reasonably suggest could make transmission more likely, in fact, did not make the slightest bit of difference.


Are women more vulnerable than men? Zero transmissions in both genders. Is anal sex riskier than vaginal sex? Zero transmissions for both. Is it riskier to be the bottom than the top? Zero transmissions for bottoms and for tops. Do sexually transmitted infections increase the risk? Zero transmissions in people with sexually transmitted infections.

We should add that 11 of the HIV-negative partners did acquire HIV, but from somebody else. Careful analysis showed that in all 11 cases, the virus was genetically different enough from their partners' virus to indicate that they must have contracted it from another person -- from outside their main relationship.

Reporting their results in the prestigious Journal of the American Medical Association last year, the researchers summed it up: "The estimate of the overall transmission rate, and the transmission rate for anal sex, was zero."

With the consequences of 58,000 unprotected sex acts already recorded, the findings are robust. In science, having a larger number of observations or study participants generally boosts the certainty with which scientists can state their conclusions. (In statistics, this is expressed through something called the 95% confidence interval).

However, because the first phase of the PARTNER study had fewer gay couples than heterosexual couples, researchers have less data on anal sex than vaginal sex. To remedy this, a second, ongoing phase of the study is collecting more data on only gay couples. There's no indication that the researchers expect this to reveal any cases of transmission during anal sex when viral load was undetectable. It's more likely to increase the certainty and scientific rigor with which the researchers can state that, with undetectability, the transmission rate during anal sex is zero.

The HPTN 052 Study

The other key scientific study is known as HPTN 052. This randomized controlled trial (perhaps the most reliable scientific method) recruited 1763 mostly heterosexual couples with mixed HIV status. The HIV-positive partners were randomly allocated either to start HIV treatment immediately or to delay treatment until their CD4 count was considerably lower.

Again, the researchers were interested in how many HIV-negative partners wound up acquiring HIV from their HIV-positive partner. The direct comparison of couples who had immediate access to HIV treatment and couples who had to wait for it tells us about the difference that providing HIV treatment makes.

Forty-six individuals acquired HIV from their primary partner during the trial, three in the immediate-treatment arm and 43 in the delayed-treatment arm. This amounts to 93% fewer transmissions in the immediate-treatment group.

But what about those three people who picked up HIV from his or her partner, even though they had been prescribed HIV treatment?

In each case, the HIV-positive partner did not have an undetectable viral load. The transmission occurred either within the first three months of taking treatment (before viral load had been durably suppressed) or after HIV treatment had failed and viral load had risen to detectable levels. "No linked infections were observed when HIV-1 infection was stably suppressed by [antiretroviral treatment] in the index participant," Myron Cohen, M.D., and colleagues reported in New England Journal of Medicine last year.

So, while the 93% figure is some way off 100% effectiveness, it relates to the reduction in transmissions when HIV treatment is provided early, not the reduction when HIV treatment results in an undetectable viral load.

Transformative Evidence

How to sum up this scientific evidence? Demetre Daskalakis, M.D., M.P.H., assistant commissioner for the Bureau of HIV/AIDS at the New York City Health Department put it like this:

Several strong lines of evidence indicate that consistent viral suppression is truly HIV prevention. Given these empiric findings, New Yorkers living with HIV who take their antiretrovirals consistently and have suppressed viral loads should be confident that their risk of HIV transmission is negligible.

According to Matthew Hodson of NAM aidsmap:

This understanding transforms the way that HIV is considered with enormous implications for what it now means to live with HIV and the best ways to prevent it. ... The fear of catching HIV from a sexual partner fuels HIV stigma, which is why it's so important that the 'undetectable equals untransmittable' message is heard and understood.

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