Estimates of the Prevalence and Incidence of Syphilis in the United States, 2018

Background: Syphilis is a genital ulcerative disease caused by the bacterium Treponema pallidum that is associated with significant complications if left untreated and can facilitate the transmission and acquisition of HIV infection. The last prevalence and incidence estimates of the burden of syphilis in the United States were for 2008.

Methods: We generate syphilis prevalence and incidence estimates for 2018 among adults aged 14 to 49 years. We fit a simple mathematical model to 2018 case report data to generate 10,000 sets of estimates for age and sex subpopulations and summarize our estimates by their median (50th percentile); uncertainty intervals are characterized by their 25th (Q1) and 75th (Q3) percentiles. We also used our methodology to reestimate 2008 prevalence and incidence estimates.

Results: In 2018, there were an estimated 156,000 (Q1, 132,000; Q3, 184,000) prevalent and 146,000 (Q1, 126,000; Q3, 170,000) incident syphilitic infections in people aged 14 to 49 years. Men accounted for roughly 70% of prevalent infections and more than 80% of incident infections. In both sexes, there were more prevalent and incident infections in 25- to 49-year-olds than 14- to 24-year-olds. Using these methods to reanalyze 2008 data, syphilis prevalence and incidence estimates have increased 164% and 175%, respectively, between 2008 and 2018.

Discussion: Although not as common as other sexually transmitted infections, syphilis should be monitored because of its devastating sequelae. As it continues to increase in frequency, it will be important for future work to continue to track its trajectory and burden.