A new study published in the journal Health Economics has raised an important public health concern, suggesting that one of the most significant breakthroughs in HIV treatment may have indirectly contributed to a resurgence of syphilis in the United States.
For much of the 20th century, syphilis was on a steady decline. Following the widespread use of penicillin and shifts in sexual behavior, reported cases fell by more than 97% between 1943 and 2000. During this same period, the HIV epidemic emerged as a major global crisis. By 1993, HIV had become the leading cause of death among Americans aged 25 to 44 years, with a disproportionate impact on men who have sex with men.
A major turning point came in 1996 with the introduction of Highly Active Antiretroviral Therapy, commonly known as HAART. This combination therapy transformed HIV from a fatal illness into a manageable chronic condition. Over the following decades, HIV-related mortality declined dramatically, reaching a historic low of 4,748 deaths in 2023. The success of HAART is widely considered one of the most important achievements in modern medicine, significantly improving both survival and quality of life for people living with HIV.
However, researchers from institutions including the University of Texas, Baylor University, and University of North Carolina report that this medical progress coincided with a reversal in syphilis trends. After decades of decline, syphilis rates began rising again, reaching a 60 year high of 62.2 cases per 100,000 people in 2022.
To better understand this pattern, the researchers analyzed long-term surveillance data from the Centers for Disease Control and Prevention along with pharmaceutical sales data. Their findings suggest that the widespread adoption of HAART may have indirectly influenced syphilis transmission through a combination of behavioral and demographic changes.
One major factor is improved survival. As people living with HIV began to live longer and healthier lives, sexual activity levels increased, including instances of unprotected sex. Longer life expectancy also extended the period during which individuals could be exposed to sexually transmitted infections, increasing the likelihood of acquiring infections such as syphilis.
The study estimates that the introduction of HAART was associated with approximately 71,190 additional syphilis cases between 1996 and 2008. Without HAART, the researchers suggest that syphilis incidence during this period could have been about 81% lower. The increase was particularly evident among men in states with historically high AIDS prevalence and greater uptake of antiretroviral therapy, while rates among women continued to decline.
The researchers note that increased longevity alone does not fully explain the trend. The data point strongly toward behavioral changes, particularly higher risk sexual practices, as a key driver. Alternative explanations, including shifts in drug use patterns, changes in social norms, or variations in public health funding, were examined but did not sufficiently account for the observed rise.
Public health experts emphasize that these findings should not be interpreted as criticism of HAART. The study highlights the need to adapt prevention strategies alongside advances in treatment. Experts recommend strengthening education on safer sexual practices, expanding routine screening for sexually transmitted infections, and integrating STI prevention more closely into HIV care programs.
Recent developments add urgency to these recommendations. Rates of congenital syphilis, which is transmitted from mother to child during pregnancy, have also increased in the United States in recent years, pointing to gaps in early detection and prenatal screening. In addition, the growing use of HIV prevention tools such as pre-exposure prophylaxis, commonly known as PrEP, has in some studies been associated with reduced condom use, which may further influence the spread of other sexually transmitted infections.
The findings highlight a broader lesson in public health. Even the most transformative medical advances can reshape human behavior and disease patterns in complex and sometimes unintended ways. As HIV care continues to improve, a parallel focus on comprehensive sexual health strategies will be essential to prevent unintended consequences and protect long term population health.
References
Beheshti D, Cunningham S, Eilam N. Unintended Consequences of Life-Saving Pharmaceutical Innovations: How HAART Led to the Resurgence of Syphilis. Health Econ. 2026 Apr 22.