[1] Ctr Dis Control & Prevent, Natl Ctr HIV STD & TB Prevent, Div Sexually Transmitted Dis Prevent, Atlanta, GA 30333 USA
[2] Univ Laval, Unite Rech Sante Populat, Hop St Sacrement, CHA, Quebec City, PQ G1K 7P4, Canada
[3] Univ London Imperial Coll Sci & Technol, Fac Med, Dept Social Sci & Med, London, England
[4] Univ London Imperial Coll Sci & Technol, Fac Med, Dept Infect Dis Epidemiol, London, England