The Spectrum-STI Groups model: syphilis prevalence trends across high-risk and lower-risk populations in Yunnan, China

Abstract

The Spectrum-STI model, structured by sub-groups within a population, was used in a workshop in Yunnan, China, to estimate provincial trends in active syphilis in 15 to 49-year-old adults. Syphilis prevalence data from female sex workers (FSW), men who have sex with men (MSM), and lower-risk women and men in Yunnan were identified through literature searches and local experts. Sources included antenatal care clinic screening, blood donor screening, HIV/STI bio-behavioural surveys, sentinel surveillance, and epidemiology studies. The 2017 provincial syphilis prevalence estimates were 0.26% (95% confidence interval 0.17-0.34%) in women and 0.28% (0.20-0.36%) in men. Estimated prevalence was 6.8-fold higher in FSW (1.69% (0.68-3.97%) than in lower-risk women (0.25% (0.18-0.35%)), and 22.7-fold higher in MSM (5.35% (2.74-12.47%) than in lower-risk men (0.24% (0.17-0.31%). For all populations, the 2017 estimates were below the 2005 estimates, but differences were not significant. In 2017 FSW and MSM together accounted for 9.3% of prevalent cases. These estimates suggest Yunnan's STI programs have kept the overall prevalence of syphilis low, but prevalence remains high in FSW and MSM. Strengthening efforts targeting FSW and MSM, and identification of other risk populations e.g. among heterosexual men, are critical to reduce syphilis.

Figure 1

Prevalence data and estimates for active syphilis, Yunnan province, 2005–2017: (a) Lower-risk women; (b) Lower-risk men: (c) Female Sex Workers; (d) Men who have sex with men. Prevalence estimates shown are after adjustment for diagnostic test performance. Solid lines are the best estimates; dotted lines the corresponding 95% confidence interval. The blood donor screening data that could not be disaggregated by sex; Table 1) were used in the estimation for lower-risk men, as this group had few other data available (panel b) and the majority of donors in this dataset (56%) were men. In contrast, they were not used in the estimation for lower-risk women, for whom there were many more data points (panel a).

Figure 2

Syphilis prevalence estimates under alternative data inclusion criteria, for (a) Lower-risk women, (b) FSW and (c) MSM. Series titles shown in panel b also apply to panel c.