Ongoing transmission of a single hepatitis B virus strain among men having sex with men in Amsterdam R. van Houdt, 1 S. M. Bruisten, 1,2 R. B. Geskus, 1,3 M. Bakker, 4 K. C. Wolthers, 4 M. Prins 1,2 and R. A. Coutinho 2,5 1 Public Health Service Amsterdam, Department of Infectious Diseases, Amsterdam, the Netherlands; 2 Department of Internal Medicine, Academic Medical Center, University of Amsterdam, CINIMA, Amsterdam, the Netherlands; 3 Department of Clinical Epidemiology, Biostatistics and Bioinformatics, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands; 4 Department of Medical Microbiology, Academic Medical Center, University of Amsterdam, CINIMA, Amsterdam, the Netherlands; and 5 Center for Infectious Disease Control, National Institute for Public Health and the Environment, RIVM, Bilthoven, the Netherlands Received February 2009; accepted for publication April 2009 SUMMARY. For the past decade, a specific hepatitis B virus (HBV) genotype A strain has been prevalent among men having sex with men (MSM) in Amsterdam, the Nether- lands. At what point in time this strain was introduced in the MSM population, and why only this specific strain continues to be transmitted, remains unclear. Between 1984 and 2003, sera of 1862 MSM were retrospectively screened for anti-HBc in the context of the Amsterdam Cohort studies. After 2003, most MSM participating in this study were vaccinated, making further testing less useful. HBV DNA from anti-HBc seroconverters was amplified and sequenced. Poisson regression was used to test for temporal trends in HBV and HIV incidence. Of the 1042 MSM who were negative for anti-HBc at entry, 64 had seroconverted during follow-up at a median age of 32. At the point of seroconversion, 31 MSM were HIV positive. HBV incidence declined dramatically in the first years and then remained stable throughout the study period. The HBV and HIV incidence ran almost in parallel. With the exception of three MSM, all were infected with genotype A. Fifteen of these (41%) were infected with an identical genotype A strain. For the past two decades, an identical genotype A strain has been circulating among MSM in the Nether- lands. Although HBV is generally considered more infec- tious than HIV, this study shows that the trend and magnitude in HBV and HIV incidence among MSM are similar. Keywords: HBV and HIV incidence, hepatitis B, men having sex with men, phylogeny. INTRODUCTION In countries where hepatitis B virus (HBV) is highly endemic, most transmissions take place at birth or during early childhood. In most western countries where HBV endemicity is low, HBV transmission is mainly restricted to risk groups, such as drug users (DUs), commercial sex workers (CSWs), and men having sex with men (MSM). The Netherlands is such a low endemic country, where sexual transmission, especially between men, is the main route of HBV infection [1,2]. The overall HBsAg prevalence in the Netherlands is estimated to be 0.3–0.5%, whereas for MSM it is estimated to be up to ten times higher [3]. Previous studies have shown that in the past decade a single genotype A strain has been circulating among MSM in Amsterdam, despite the introduction of a nationwide vacci- nation program also targeted at MSM [4,5]. At what point in time this strain was introduced in the MSM population, remains unclear. As yet there is also no explanation regarding why only this specific genotype A strain continues to be transmitted. To obtain more insight into the circulation of this geno- type A strain among MSM, a retrospective study was undertaken of MSM who participated in the Amsterdam Cohort Studies (ACS). The ACS is an ongoing, prospective, open cohort of MSM initiated in 1984 [6]. To obtain better insight into the strains that are actually being transmitted, only strains from MSM who had seroconverted for anti-HBc during follow-up were included in the phylogenetic analysis. The S-gene (672 nt.) of those who had seroconverted to anti- HBc during follow-up was amplified and sequenced. HBV incidence is often estimated or based on the reported number of acute HBV infections. The significant number of Abbreviations: ACS, Amsterdam cohort studies; CSWs, commercial sex workers; DUs, drug users; HBV, hepatitis B virus; MSM, men having sex with men; STI, sexually transmitted infections. Correspondence: Robin van Houdt, Department of Infectious Dis- eases, Public Health Service Amsterdam, Postbus 2200, 1000 CE Amsterdam, the Netherlands. E-mail: rvhoudt@ggd.amsterdam.nl. Journal of Viral Hepatitis, 2010, 17, 108–114 doi:10.1111/j.1365-2893.2009.01158.x Ó 2009 Blackwell Publishing Ltd