Who Is at Highest Risk of Transmitting HIV?

3 in 5 infections attributed to people aware of their status

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On a case-by-case basis, clinicians are often able to assess who is at greater risk for transmitting HIV based on such factors as the person's viral load, risk behaviors, illicit drug use, etc. By actively addressing these factors, the risk to the individual can be significantly minimized.

From a population-based perspective, researchers are increasingly able to do the same by quantifying transmission risk based on real-world, epidemiological data. What they have found is that one factor alone, more than anything else, is associated with lower transmission risk: being in and staying in medical care.

In fact, even without antiretroviral therapy (ART), regularly seeing a doctor more than halves the risk of transmitting HIV compared to someone who is diagnosed.

HIV Transmission Risk by Stage of Care

Researchers from the Centers for Disease Control and Prevention (CDC) in Atlanta recently performed a mathematical modeling using data from the National HIV Surveillance System and their Medical Monitoring Project (MMP).

The study aimed to determine transmission rates based both on where an individual stood on the so-called HIV Care Continuum (i.e., whether they were tested, in care, on treatment, etc.) and the various risk factors that may place that person at higher or lower risk of transmission.

It is estimated that around 1,150,000 people with HIV are living in the U.S. today. Of those, approximately:

  • 82 percent are diagnosed
  • 37 percent are linked and retained in care
  • 33 percent are on ART
  • 25 percent are virally suppressed with an undetectable viral load

By referencing these statistics with transmission surveillance data and random samplings from the MMP, the CDC researchers were able to establish that the overall transmission rate in the U.S. is 3.9 transmissions per 100 individuals per year (100PYs).

Breaking that figure down by stage of care, the researchers were able to conclude that:

  • People undiagnosed for HIV had a transmission rate of 6.6 infections per 100PYs (nearly twice the overall average).
  • People who were diagnosed—who simply knew their status, irrespective of ART—transmitted 5.3 infections per 100PYs.
  • People who were diagnosed and retained in care transmitted 2.6 infections per 100PYs.
  • People prescribed ART who were not fully suppressed transmitted 1.8 infections per 100PYs.
  • People prescribed ART who were virally suppressed transmitted only 0.4 infections per 100PYs.

From a demographic standpoint, the majority of transmissions (58 percent) were seen in men who had sex with men (MSM). And while adults age 35-44 accounted for 34 percent of all transmissions, the highest rate of transmission (6.2 infections per 100PYs) occurred among adults 25-34.

So What Does This Tell Us?

From a real-world perspective, these figures confirm that patients with sustained, undetectable viral loads are likely to have a 94 percent reduction in the likelihood of transmission compared to those who are not diagnosed. [Placing the uninfected partner on HIV pre-exposure prophylaxis (PrEP) can even further reduce that risk.]

On the flip side, the figures also suggest that, while undiagnosed patients represent 18 percent of the total HIV population in the U.S., they account for more than 30 percent of all infections. Worse yet, individuals who are fully aware of their status yet remain unlinked to care account for a staggering three out of every five new infections.

MMP modeling further supports these statistics by concluding that 62 percent of those undiagnosed regularly engage in condom-less sex. By contrast, only 16 percent of people on ART reported condom-less sex, while even fewer (10.5 percent) avoided condoms if they were fully undetectable.

Similarly, patients in care and those on ART reported having two and three sexual partners in the previous year, respectively, compared to those who were either undiagnosed or diagnosed without care (8 to 8.8 partners). The same results were in seen in patients who injected drugs, with only 0.9 percent of those on ART reporting injection drug use compared to 6.3 percent who were undiagnosed.

Ultimately, the take-home message is simple: being in and staying in care represents the highest reduction in HIV risk overall, more than halving the likelihood of transmission while taking HIV therapy, irrespective of viral load or regimen type, reduces risk by a further 30 percent.

In fact, just getting tested for HIV can reduce a person's transmission risk by some 19 percent, suggesting that awareness alone can, to some degree, alter an individual's risk behaviors. It certainly a good place to start.

To find an HIV testing site or access HIV care services nearest you, visit the AIDSVu locator site operated by the Rollins School of Public Health at Emory University, or contact the HIV/AIDS hotline in your area, many of which are available 24 hours a day.

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