The authors’ affiliations are as follows: the Gladstone Institute of Virology and Immunology (R.M.G., V.M., P.G., R.J.H., J.J.M., P.D., J.L.), the University of California, San Francisco (R.M.G., A.Y.L., S.P.B., K.M., F.W., D.V.G.), and the HIV Research Section, San Francisco Department of Public Health (A.Y.L., S.P.B.) — all in San Francisco; Investigaciones Medicas en Salud, Lima (J.R.L., L.V.), Asociación Civil Impacta Salud y Educación, Lima (J.R.L., J.V.G.-C., M.E.R.-C., C.G.), and Asociación Civil Selva Amazónica, Iquitos (M.C.) — all in Peru; the University of Colorado, Denver (P.L.A., L.R.B., J.-H.Z.).; Fundación Ecuatoriana Equidad, Guayaquil, Ecuador (O.M.- H., T.F.); Fundação Oswaldo Cruz, Instituto Nacional de Infectologia Evandro Chagas, Laboratório de Pesquisa Clínica em DST/AIDS (V.G.V.), and Projeto Praça Onze, Hospital Escola São Francisco de Assis, Universidade Federal do Rio de Janeiro (M.S.) — both in Rio de Janeiro; Brown University, Providence, RI (K.H.M.); the Fenway Institute, Fenway Health, Boston (K.H.M.); the Division of Clinical Immunology and Allergy, School of Medicine, University of São Paulo, and Instituto de Investigação em Imunologia — both in São Paulo (E.G.K.); Research Institute for Health Sciences, Chiang Mai University, Chiang Mai, Thailand (S.C.); Desmond Tutu HIV Centre and Department of Medicine, University of Cape Town, Cape Town, South Africa (L.-G.B.); Division of AIDS, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD (A.I.M., D.N.B.); Gilead Sciences, Foster City, CA ( J.F.R., H.S.J.); DF/Net Research, Seattle (B.P.); Applied Health Research, Brighton, MI (K.R.A.); and the Center for Health, Intervention, and Prevention, University of Connecticut, Storrs (K.R.A.).Antiretroviral chemoprophylaxis before exposure is a promising approach for the prevention of human immunodeficiency virus (HIV) acquisition.