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As background for their study, the CDC team, led by Dr. Pragna Patel, noted that effective HIV-prevention programs rely on accurate estimates of the per-act risk of HIV acquisition from different types of exposure to the virus (e.g., sexual exposure, transfusions, mother-to-child transmission, injection-related). To improve existing estimates, the team reviewed well over 15,000 articles on per-act HIV transmission risk and potentially modifying factors, including condom use, male circumcision, and antiretroviral therapy. They then performed sophisticated statistical analyses on the relevant data to find the estimated risk of HIV transmission for specific types of exposure. (CDC has a chart showing the relative risks of different exposures.)
Their data analysis focused primarily on the odds of transmission when risk-reduction methods (such as HIV medications, PrEP, condoms, etc.) were not being used and cofactors for transmission (such as other sexually transmitted infections) were not present.
Overall, they found that receiving a transfusion with HIV-infected blood has the greatest risk of transmission, followed by transmission from a mother to her child during pregnancy, labor and delivery, and breastfeeding. The only sexual behavior that was riskier than injection drug use was receptive anal intercourse, which was estimated to pose a risk that was two times greater than that of acquiring HIV through injection with a syringe that has been used by someone with HIV.
The team found that sexual exposure risks ranged from a number so low for oral sex that it could not be reliably estimated to 138 infections per 10,000 exposures for receptive anal intercourse. The paper also addressed the effectiveness of available prevention options used as a single strategy and in combination with each other.
This study was an important addition to our knowledge about HIV risk and prevention. The fact that it has been so widely cited by other researchers is a mark of the quality of CDC’s work and the relevance of the information to HIV-prevention efforts—both in the United States and globally.
The study also illustrates the continuing need for these types of data, which are essential to HIV prevention and care, and the importance of updating the data regularly so that they can be used to make informed choices that are based on the most accurate information at any given time.
For the most part, our knowledge of these issues remains relatively stable and little changes for years. Recently, however, we have experienced more rapid changes in our understanding of how HIV and other infectious diseases are transmitted. As we continue to improve our knowledge of how HIV is acquired, studies like this one help us develop more effective prevention strategies and interventions. More accurate estimates of transmission risks allow us to tailor our messaging and outreach to key populations and increase the impact of our efforts to end the HIV epidemic.
In just a couple of years, our knowledge about the effect of HIV treatment that results in viral load suppression has changed significantly. Since the CDC article was published in 2014, we have gathered both new and additional evidence that people living with HIV who take their HIV medications as prescribed and achieve long-term viral suppression have essentially no risk of transmitting the virus to their sexual partners (“treatment as prevention”). We also know that pre-exposure prophylaxis (PrEP) is remarkably effective in preventing new HIV infections among individuals at greatest risk when they take it as prescribed.
Keeping up with the rapidly changing science can be challenging. But doing so means that people are more able to accurately understand their risks and how those risks add up over time. That gives them the ability to make fully informed decisions about the approaches that line up best with their own preferences (and those of their partners), while at the same time giving them the ability to protect themselves and their loved ones from HIV.
At some point—probably soon—there will be another meta-analysis of the per-act risk of HIV transmission, and it will incorporate what we know now about treatment as prevention and PrEP. No matter who does the next analysis, the CDC team has set the gold standard for high-quality statistical work. Congratulations to Dr. Patel and her co-authors, Dr. Craig B. Borkowf, Dr. John T. Brooks, Dr. Arielle Lasry, Dr. Amy Lansky, and Dr. Jonathan Mermin!