EPIDEMIOLOGY AND SOCIAL SCIENCE Behavioral Impact, Acceptability, and HIV Incidence Among Homosexual Men With Access to Postexposure Chemoprophylaxis for HIV Mauro Schechter, MD, PhD,* Regina F. do Lago, MPH,* Aaron B. Mendelsohn, PhD,† Ronaldo I. Moreira, PhD,* Lawrence H. Moulton, PhD,‡ and Lee H. Harrison, MD,§ for the Prac ¸a Onze Study Team Background: Little is known about the behavioral impact, accept- ability, and incidence of HIV infection in persons with easy access to post–sexual exposure prophylaxis (PEP) to prevent HIV. Methods: Participants were recruited from a well-characterized, high-risk HIV seronegative homosexual male cohort in Rio de Janei- ro, Brazil, given a 4-day supply of zidovudine and lamivudine, and instructed to begin PEP immediately after an eligible exposure. For eligible exposures, an additional 24-day supply was provided. Re- ported behavior, PEP utilization, adverse events, and incident HIV infection were the main study outcomes. The observed and expected incidences of HIV infection were compared. Results: Two hundred subjects were enrolled and followed for a median of 24.2 months. The median age was 28 years. PEP was ini- tiated 109 times by 68 participants (34.0%). In comparison to reported behavior at baseline, reported high-risk sexual activities on average declined over time for both PEP and non-PEP users. There were no serious drug-related adverse events. There were 11 HIV seroconver- sions, 10 among non-PEP users and 1 that was a PEP failure. The overall seroincidence was 2.9 per 100 person-years (95% CI = 1.4, 5.1). The expected number of new HIV infections and corresponding expected seroincidence based on the risk profile were 11.8 and 3.1, respectively (P > 0.97). The most commonly reported reasons for not initiating PEP among seroconverters were sex with a steady partner and not considering the exposure to be of sufficiently high risk to warrant PEP. Conclusion: PEP was safe and did not appear to be associated with increases in reported high-risk behavior in our cohort. Ready access to PEP did not appear to substantially affect HIV transmission, suggest- ing a limited public health impact of this intervention. Key Words: postexposure chemoprophylaxis, postexposure prophy- laxis, HIV, HIV incidence, lamivudine, zidovudine (J Acquir Immune Defic Syndr 2004;35:519–525) P ostexposure chemoprophylaxis (PEP) to prevent HIV in- fection is widely used in a variety of settings. In one retro- spective case-control study of health care personnel, postex- posure treatment with zidovudine was associated with an esti- mated 81% reduction in the risk of HIV infection, although the lack of a randomized trial does not allow firm conclusions about this intervention. 1 The efficacy of antiretroviral therapy in reducing maternal-infant transmission of HIV infection is also high, with some of the effect appearing to be due to post- partum chemoprophylaxis of the infant. 2–4 There are no data on the efficacy of chemoprophylaxis following sexual exposure to HIV, and animal studies showed mixed results. 5,6 Despite this lack of information, PEP use af- ter high-risk sexual exposures is advocated and often used, 7–11 based primarily on data from the other settings in which it has been studied. However, the health care setting is usually asso- ciated with a single exposure, which makes the justification for providing chemoprophylaxis and compliance with the postex- posure regimen relatively straightforward. This is in contrast to post–sexual exposure prophylaxis, for which the possibility of repeated exposures is more likely. The use of chemoprophy- Received for publication May 13, 2003; accepted November 26, 2003. From *Infectious Diseases Service, Hospital Universitário Clementino Fraga Filho; Department of Preventive Medicine, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil; †Department of Epidemiology, Univer- sity of Pittsburgh Graduate School of Public Health, Pittsburgh, PA; ‡De- partment of International Health and Department of Biostatistics, Johns Hopkins University, Bloomberg School of Public Health, Baltimore, MD; and §Infectious Diseases Epidemiology Research Unit, University of Pittsburgh Graduate School of Public Health and School of Medicine, Pittsburgh, PA. Support for this study was provided by a grant from GlaxoSmithKline; by a grant from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) to Dr. Schechter; by a Research Career Award (K24 AI52788), National Institute of Allergy and Infectious Diseases, to Dr. Harrison; and by a grant from the Fogarty International Center Fogarty (3 D43 TW01038), National Institutes of Health. Dr. Schechter has re- ceived honoraria from GlaxoSmithKline for giving talks. Reprints: Mauro Schechter, AIDS Research Laboratory, Hospital Univer- sitário Clementino Fraga Filho, Universidade Federal do Rio de Janeiro, Av. Brig. Trompowsky, Ilha do Fundão, Rio de Janeiro, CEP: 21941-590, Brazil (e-mail: maurosch@hucff.ufrj.br). Copyright © 2004 by Lippincott Williams & Wilkins J Acquir Immune Defic Syndr • Volume 35, Number 5, April 15 2004 519