Background Aggressive efforts to reduce the viral load among persons with HIV infection, including early diagnosis, linkage to care, and early initiation of antiretroviral treatment may significantly reduce HIV transmission at the community level, and thus becoming a major focus of HIV prevention policy. However, it is not known how this approach will affect HIV risk behaviours and the epidemiology of non-HIV sexually transmitted infections (STI).

Objective To study trends in HIV, early syphilis, and high-risk behaviours among men who have sex with men (MSM) in Denver.

Methods We analysed and triangulated data from 3 sources: the Denver Metro Health (STI) Clinic; Denver STI surveillance; Thee Denver MSM waves of the National HIV Behavioral Surveillance system (NHBS).

Results Among MSM visiting DMHC, the rate of new HIV diagnoses decreased from 3.3% in 2007 to 2.5% in 2012 (–24%) while the rate of early syphilis increased from 1.3% to 2.5% (+92%) during the same time period. Similarly, in Denver (all populations), rates of newly reported HIV infections decreased from 32.4/100,000 in 2007 to 25.3/100,000 in 2011 (–22%), but newly reported cases of primary and secondary syphilis increased from 5.3/100,000 to 11.8/100,000 (+123%) during the same time period. Finally, Denver NHBS data showed the proportion of MSM reporting unprotected anal sex stable at 36%–37% between 2005, 2009 and 2011 data waves.

Conclusion Despite continued high levels of risk behaviours among Denver MSM, the rate of new HIV infections is decreasing, providing evidence for the effectiveness of efforts to reduce the community HIV viral load. However, at the same time, rates of early syphilis continue to rise. Without simultaneous efforts to reduce risk behaviours, a biomedical approach to reduce the community HIV viral load may result in gains for HIV prevention but losses in the prevention of syphilis and other STI.