1 aVirology Unit, Department of Biomedical Sciences bUnit of Epidemiology and Control of HIV/STD, Department of Public Health cHIV/STD Clinic, Department of Clinical Sciences, Institute of Tropical Medicine, Antwerp, Belgium dVic Arendt, Sylvie Delhalle, Centre de Recherche Public de la Santé, Luxemburg, Luxemburg eUnité de Régulation des Infections Rétrovirales, Institut Pasteur, Paris, France.