Background:  For many years, hepatitis C virus (HCV) infection was considered a disease of intravenous drug users (IDU) and hemophiliacs. Recently, there have been several outbreaks of HCV infections among men who have sex with men (MSM) in large European cities. However, the impact of these localized epidemics on the incidence of HCV infections at the population level has not been described before. We assessed changes in the HCV infection incidence in IDU and MSM in the Swiss HIV Cohort Study (SHCS) during the last 12 years.

Methods:  HCV infection incidence was assessed from 1998 (when routine HCV screening every 2 years was introduced in the SHCS) until August 2011. The presumed HIV transmission mode was used, define patients as IDU or MSM. All HCV^– IDU and MSM who had at least 1 follow-up HCV antibody measurement after July 1998 were included. MSM with a history of IDU and patients with HCV seroconversion before 1998 were excluded. Incidence rates (IR) were determined as seroconversions per 100 person-years. Individual follow-up ended at the time of the first positive or last negative HCV serology. We compared yearly HCV infection incidence over time between IDU and MSM by calculating IR from a Poisson model.

Results:  Of 4402 MSM and 2660 IDU screened for HCV infection, 3255 (74%) and 122 (5%) had a negative HCV serology at baseline and available serological follow-up. Over a total follow-up time of 22, 905 person-years for the MSM group and 705 person-years for the IDU group, 91 (3%) and 39 (32%) seroconverted, respectively. The figure shows yearly HCV infection IR by transmission group between 1998 and 2011. During this period, the IR of HCV infections in IDU decreased from 13.5 (95%CI 7.8 to 22.0) per 100 person-years in 1998, 1.1 (0.2, 5.7) in 2011, with only 1 incident case observed in the last 3 years. In the MSM group, the IR increased from 0.2 (0.1 to 0.6) per 100 person-years in 1998, 7.4 (3.9 to 13.0) in 2011, with 41 cases observed in the last 3 years. For comparison, the yearly IR of HCV infections in heterosexual patients remained <0.8 per 100 person-years, with only 1 case in 2011 (IR 0.7 per 100 person-years, 95%CI 0.1 to 3.2) during a total follow-up of 20,789 patient-years.

Conclusions:  In the SHCS, the large majority of recent HCV infections occurred in MSM. In the last 12 years, HCV incidence decreased in HIV⁺ IDU and markedly increased in MSM. This observation underscores the need for improved HCV surveillance and prevention among HIV⁺ MSM.