(262) Evaluation of the association between primary and secondary syphilis incidence and social vulnerability index

Objective:
To examine the association between Social Vulnerability Index (SVI) and primary and secondary (P&S) syphilis incidence in pregnant individuals across the United States (US) over time.

Study Design:

This was a cross-sectional study using the Centers for Disease Control and Prevention Natality dataset. Subjects were categorized into quartiles (Qs) based on county-level SVI, with Q4 being the most vulnerable. The primary outcome was P&S syphilis incidence per 100,000 births. Mixed-effect generalized linear models with negative binomial distribution were used to quantify disparities in syphilis incidence. Spatial and temporal patterns across quartiles and years were mapped to US counties and visualized according to SVI. A difference-in-differences (DID) analysis was used to compare trends in syphilis incidence across different SVI quartiles and time periods.

Results:
The incidence of P&S syphilis in the US increased by approximately 25,000 from 2016 to 2021. Rates of syphilis increase from 2016 to 2021 at the county-level are plotted on a U.S. map, showing clusters of spread, with the highest rates being in the southwest US (Figure 1). The estimated incidence of syphilis across all years was graphed for each SVI quartile, showing both the increasing disparity of syphilis rates between SVI quartiles across 2016-2021, and the consistently higher rates of syphilis in more socially vulnerable areas (Figure 2).
The most significant widening disparity was observed between Q4 and Q1, with a difference of incidence increase by 85 cases per 100,000 births in Q4 (95% CI [50, 120]; p = 0.003). Similarly, the incidence rate in Q3 had a widening disparity compared to Q2 (DID estimate 35 cases per 100,000 births; 95% CI [6, 67]; p = 0.0028). There was no statistically significant difference between SVI Q2 and Q1.


Conclusion:
P&S syphilis incidence rates increased disproportionately amongst counties with higher social vulnerability, emphasizing the need for targeted interventions geared towards these communities.