[1] Univ Calif San Francisco, Dept Dermatol, 2340 Sutter St,N421, San Francisco, CA 94115 USA
[2] London Sch Hyg & Trop Med, Fac Epidemiol & Populat Hlth, Keppel St, London WC1E 7HT, England
[3] Univ Penn, Perelman Sch Med, Dept Dermatol, 3400 Civ Ctr Blvd, Philadelphia, PA 19104 USA
[4] Univ Penn, Perelman Sch Med, Dept Biostat & Epidemiol, 3400 Civ Ctr Blvd, Philadelphia, PA 19104 USA