The impact of syphilis screening among female sex workers in China: a modelling study

Abstract

Background: In China, female sex workers (FSWs) are at high risk of syphilis infection, but are hard to reach for interventions. Point-of-care testing introduces opportunities for expanding syphilis control measures. Modelling is used to estimate the impact of using rapid tests to screen FSWs for syphilis. In other settings, modelling has predicted large rebounds in infectious syphilis following screening, which may undermine any impact achieved.

Methods: A deterministic syphilis transmission model among FSWs and clients was fitted to data from Yunnan Province (FSW syphilis prevalence = 7.5%), and used to estimate the impact of rapid syphilis testing and treatment for FSWs. Impact projections were compared for different model structures that included risk heterogeneity amongst FSWs, incoming syphilis infections amongst new FSWs and clients and re-infection from FSWs' regular non-commercial partners. The rebound in syphilis prevalence after screening ceased was explored.

Results: All model structures suggest yearly syphilis screening could substantially reduce (by 72-88%) syphilis prevalence amongst FSWs in this setting over five years. However, incoming syphilis infections amongst new FSWs and clients or re-infections from regular non-commercial partners of FSWs can considerably reduce (>30%) the proportion of infections averted. Including heterogeneity in risk amongst FSWs had little effect upon the proportion of infections averted. In this setting, the rebound in syphilis prevalence after screening ceased is predicted to be slight, but it could be large in high prevalence settings.

Conclusions: Rapid test screening could dramatically reduce syphilis prevalence amongst hard-to-reach groups, but strategies to reduce re-infection from regular non-commercial partners are needed to maximise impact.

Figure 1. Model structure: syphilis stages and transitions.

Schematic diagram showing different stages of infection for infected FSWs, clients and regular partners, and transitions between different stages. Leaving rates are not shown ( leaving every compartment). Solid lines show normal transitions in the natural history, dashed lines show transitions due to treatment.

Figure 2. Projected intervention impact for the baseline model.

The baseline model is model 3, which has a heterogeneous FSW population, incoming syphilis infection and no regular partners. A test with sensitivity of 87% was used, with FSWs being tested on average once per year, and with immediate treatment of all individuals testing positive. Results are shown over 5 years of the intervention and for an additional 10 years after the intervention stopped. Panels (A) and (B) summarise the range of prevalences seen across the different fits (N = 326) for (A) FSWs and (B) clients. The thick solid line shows the median, the dark shaded area shows the interquartile range (25^th–75^th percentile), the light shaded area the full range (minimum-maximum), and the dashed line shows the best fit. The black circles with error bars represent the data (mean and range) that the model was fit to. In panels (C) and (D) impact is presented as (C) percentage change in prevalence (compared to pre-intervention endemic levels) and (D) percentage infections averted since the start of the intervention (compared with the situation where there was no intervention), and these are shown at yearly intervals. The thick horizontal line in each box is the median, with the box limits denoting the 25^th and 75^th percentiles and the whiskers denoting the 2.5^th and 97.5^th percentiles. The dotted vertical lines mark the start and end of the intervention.

Figure 3. Effect of screening coverage on intervention impact after 5 years of intervention.

Results are shown for both FSWs and their clients for all of the fits (N = 326) for model 3 (heterogeneous FSW population, incoming syphilis infection, no regular partners). A test with sensitivity of 87% was used, with immediate treatment of all individuals testing positive. Impact is presented as (A) relative reduction in prevalence (compared to pre-intervention levels) and (B) percentage infections averted since the start of the intervention (compared with the situation where there was no intervention). The thick horizontal line in each box is the median, with the box limits denoting the 25^th and 75^th percentiles and the whiskers denoting the 2.5^th and 97.5^th percentiles.

Figure 4. Timing and height of peak rebound in infectious syphilis for different FSW syphilis prevalence levels.

Results are shown for model 3 using the best fit parameter set and varying (transmission probability per act) between 0.01 and 1 to produce different epidemic settings. A 5-year intervention with yearly testing of all FSWs, using a rapid test of 87% sensitivity, was simulated. The x-axis shows overall pre-intervention syphilis prevalence (all infected stages) in the FSW population (high risk+low risk), and the rebound statistics shown are for infectious syphilis (primary, secondary and recurrent secondary stages) in the total FSW population (high risk+low risk).

Figure 5. Overtreatment and treatment efficiency in model 3 at different intervention coverage.

(A)% of treatments administered over the last month which were correct (i.e.% of those treated that have current infection rather than previous infection) over the course of a 5-year intervention using a rapid test with 87% sensitivity with a testing interval of 1 or 4 years as indicated, using model structure 3 (heterogeneous FSW population, incoming syphilis infection, no regular partners), (B) efficiency of treatment (total number of infections averted in FSWs and clients per treatment administered) over the last month, over this same period.

Figure 6. Impact of intervention on prevalence and percentage of infections averted for different model structures.

Impact is shown at three different time points for FSWs (A,C) and clients (B,D). Impact is presented as relative change in prevalence (compared to pre-intervention levels) (A,B) and percentage infections averted since the start of the intervention (compared with the situation where there was no intervention) (C,D). Simulated intervention assumed FSW were screened once per year with a rapid test of 87% sensitivity, with all individuals testing positive receiving immediate treatment. The thick horizontal line in each box is the median, with the box limits denoting the 25^th and 75^th percentiles and the whiskers denoting the 2.5^th and 97.5^th percentiles. Impact is shown at 6 months, 5 years and 10 years after the start of a 5-year intervention (so that 10 years is 5 years after the end of the intervention). The different population models are: (1) baseline-homogeneous FSW population with no syphilis infection among FSWs and clients; (2) heterogeneous FSW population with no infection in new FSWs and clients; (3) heterogeneous FSW population with syphilis infection in both new FSWs and new clients; (4) heterogeneous FSW population with incoming syphilis infection and regular partners of FSW included.