Epidemiological analysis of syphilis trends, disparities, and public health implications in the United States, 2018–2022 | BMC Infectious Diseases

Trends in common sexually transmitted infections in the United States, 2018–2022

Recent epidemiological trends highlight a shifting and increasingly concerning burden of STIs across the United States. Chlamydia and gonorrhea continue to be among the most commonly reported communicable diseases nationwide. While chlamydia demonstrated an overall decline of 6.20% over the past five years, it remained relatively stable with a marginal increase of 0.30% in the most recent year (Fig. 1).

Gonorrhea, in contrast, exhibited a five-year increase of 11.10%, despite an 8.70% decrease from 2021 to 2022 (Fig. 1) Syphilis displayed the most dramatic change, with a 78.90% increase over five years and a 17.00% rise in just the last year alone (Fig. 1 and Supplementary Figure S1).

Fig. 1

Percent change in reported cases of common Sexually Transmitted Infections (STIs) in the United States, 2018–2022 and 2021–2022

One-year (2021–2022) and five-year (2018–2022) percent changes in reported cases of chlamydia, gonorrhea, and syphilis based on CDC surveillance data. Percent change was calculated relative to case counts in 2018 and 2021, respectively. Syphilis demonstrated the most substantial increase over both time periods. Statistical significance of changes in syphilis rates was confirmed using a z-test for difference in proportions (p < 0.001 for both comparisons).

Geographic distribution of syphilis cases across the United States

Geographic analysis revealed that the highest rates of syphilis including both primary and secondary syphilis as well as congenital syphilis, were concentrated in the Midwest and Southern regions of the United States (Fig. 2). Among these, South Dakota was disproportionately affected, reporting the highest rate of primary and secondary syphilis during the study period.

These findings underscore the need for targeted interventions that account for regional variations in resource availability, healthcare delivery, and social determinants of health.

Choropleth map illustrating primary and secondary syphilis rates by state in 2022 based on CDC surveillance data. Darker shading indicates higher rates of syphilis per 100,000 population.

Sex- and age-related patterns of syphilis incidence

Between 2018 and 2022, syphilis cases were consistently higher among males compared to females, reflecting a greater reported burden of infection among males during this period (Fig. 5A). While both sexes experienced an upward trend in case counts over the five-year span, males consistently accounted for a disproportionately greater number of cases. Notably, the number of syphilis cases among females nearly doubled, reflecting a concerning rise in incidence within this group as well.

To further evaluate these trends, syphilis rates were averaged across the five-year period and stratified by age group for each sex. Among both males and females, the 25–29-year-old age group exhibited the highest rates of syphilis during the study period (Fig. 3B and C) In females, greater variability in case rates was observed beginning at age 20, as indicated by wider standard deviations, suggesting potential fluctuations in exposure or testing practices over time.

Racial and ethnic disparities in syphilis rates

While all racial and ethnic groups experienced increases in syphilis cases between 2018 and 2022, certain populations bore a disproportionately higher burden of infection. Individuals identifying as Black/African American and American Indian/Alaska Native (AI/AN) exhibited comparable five-year average rates of syphilis. However, in 2022, AI/AN individuals had the highest reported infection rate, which significantly influenced the overall average for this group (Fig. 4).

Odds ratio analysis in 2022 demonstrated that AI/AN individuals had the highest likelihood of syphilis infection compared to White Americans (OR: 17.87; 95% CI: 14.01–22.80; p < 0.001), followed by Black/African Americans (OR: 7.03; 95% CI: 5.53–8.94; p < 0.001) (Table 1). All racial and ethnic groups showed significantly elevated odds of infection relative to White Americans, except those identifying as Asian, who had a significantly lower odds of infection (OR: 0.45; 95% CI: 0.32–0.65; p < 0.001).

Table 1 Odds ratios (OR) for syphilis infection by race and ethnicity, United States, 2022

Syphilis prevalence by sexual behavior characteristics

Between 2018 and 2022, individuals reporting sexual activity with anonymous partners consistently exhibited the highest rates of syphilis, with this behavior associated with the greatest risk of infection in 2022 (OR: 13.1; 95% CI: 9.70–17.72; p < 0.001) (Fig. 5; Table 2). Engaging in sexual activity while intoxicated was also strongly associated with increased risk (OR: 9.49; 95% CI: 6.99–12.83; p < 0.001). In comparison, having sex with a person who injects drugs was associated with a more modest increase in risk (OR: 1.68; 95% CI: 1.17–2.37; p < 0.001).

When evaluated against the reference category of transactional sex (defined as exchanging sex for drugs or money), all other behaviors, including anonymous sex, sex while intoxicated or high on drugs, and sex with a person who injects drugs, were associated with significantly higher odds of syphilis infection.

Moreover, all four behavior categories demonstrated a general upward trend in reported syphilis cases over the five-year period, with certain behaviors, particularly anonymous sexual activity and intoxicated sex, showing more pronounced increases (Fig. 5).

Table 2 Odds ratios (OR) for syphilis infection by reported sexual behaviors, united states, 2022

Association between substance use and syphilis infection

All substance use behaviors depict an increasing trend over the five-year span from 2018 to 2022, further suggesting an increasing prevalence of syphilis (Fig. 6). Individuals who use crack have 38% lower odds of syphilis infection compared to cocaine users (OR: 0.38, 95% CI: 0.35–0.42, P < 0.001) (Table 3). Similarly, heroin use is associated with 43% lower odds of syphilis infection (OR: 0.57, 95% CI: 0.52–0.62, P < 0.001). In contrast, injection drug use is linked to an 89% increase in the odds of syphilis infection (OR: 1.89, 95% CI: 1.77–2.02, P < 0.001). Methamphetamine use shows the strongest association, with users experiencing over three times higher odds of syphilis infection compared to cocaine users (OR: 3.34, 95% CI: 3.13–3.55, P < 0.001). These findings underscore the varying risks of syphilis infection among different substance users, with injection drug use and methamphetamine use being particularly associated with elevated risk.

Table 3 Syphilis OR for substance use in 2022

Syphilis incidence by sex of sexual partner

When stratified by reported sex of sexual partners, men who have sex with men (MSM) consistently accounted for the highest number of syphilis cases from 2018 to 2022 (Fig. 7). While syphilis cases among MSM remained relatively stable over this period, greater year-to-year fluctuations were observed among men who have sex with women only (MSW), men with unknown sex of partners (MSU), and women. Notably, syphilis cases among women showed the greatest variability across the five-year period. The consistently high burden of syphilis among MSM underscores a persistent disparity in this population, which remains a critical focus for targeted public health interventions.