[1] MRC, Biostat Unit, Cambridge CB2 2SR, England
[2] Hop St Pierre & Erasme, Div Infect Dis, Brussels, Belgium
[3] State Serum Inst, Dept Epidemiol, Copenhagen, Denmark
[4] Bispebjerg Hosp, Dept Dermatovenerol, DK-2400 Copenhagen, Denmark
[5] Mortimer Market Ctr, London, England
[6] Univ Helsinki, Cent Hosp, Helsinki, Finland
[7] Natl Publ Hlth Inst, Helsinki, Finland
[8] Athens Sch Publ Hlth, Athens, Greece
[9] Olafiaklinikken, Oslo, Norway
[10] Glasgow Royal Infirm, Dept Genitourinary Med, Glasgow G4 0SF, Lanark, Scotland
[11] Natl Publ Hlth Inst, Stockholm, Sweden