[1] MRC, Programme AIDS Uganda, Inst Virus Res, Entebbe, Uganda
[2] UCL, Sch Med, Mortimer Market Ctr, London W1N 8AA, England
[3] Univ Amsterdam, Acad Med Ctr, Dept Social Med, NL-1012 WX Amsterdam, Netherlands
[4] Inst Trop Med Prince Leopold, Dept Microbiol, B-2000 Antwerp, Belgium
[5] CDC, Viral Immunol Sect, Atlanta, GA 30333 USA