The difference between HIV and syphilis prevalence and incidence cases: results from a cohort study in Nanjing, China, 2008-2010.

The available estimates of incidence and prevalence of syphilis among men who have sex with men (MSM) in Mainland China are high. We used respondent-driven sampling to recruit MSM in the study population. The participants were followed up to monitor the incidence and change of risk behaviours. A face-to-face interview was used to collect information about high-risk behaviours, demographics and recreational drug use. To test the difference between prevalent and incident cases, two nested matched case-control studies were carried out. The cases were the HIV or syphilis positives found at baseline and during follow-up. We used density sampling to sample six controls for each case. Our results indicate that compared to incident cases, prevalent cases had a higher proportion of reported unprotected anal intercourse for both HIV and syphilis. Regression analysis indicated that unprotected anal intercourse was the main risk factor among HIV-prevalent cases but not in HIV-incident cases. These differences could possibly be explained by the implementation of the risk reduction interventions. Syphilis was not a risk factor for HIV-prevalent cases but was highly associated with HIV-incident cases. Tailored interventions addressing unprotected anal intercourse and other risk factors can help to reduce the prevalence and incidence of HIV and syphilis.