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. [ABSTRACT FROM AUTHOR]