[1] Johns Hopkins Univ, Sch Hyg & Publ Hlth, W Harry Feinstone Dept Mol Microbiol & Immunol, Baltimore, MD 21205 USA
[2] Johns Hopkins Univ, Sch Med, Dept Med, Baltimore, MD 21205 USA
[3] Univ Lusaka, Teaching Hosp, Virol Lab, Lusaka, Zambia
[4] London Sch Hyg & Trop Med, Dept Infect & Trop Dis, London WC1, England