[1] Columbia Univ, Mailman Sch Publ Hlth, Ctr Infect & Immun, New York, NY 10027 USA
[2] Natl Inst Communicable Dis, Special Pathogens Unit, Natl Hlth Lab Serv, Johannesburg, South Africa
[3] Ctr Dis Control & Prevent, Div Viral & Rickettsial Dis, Special Pathogens Branch, Atlanta, GA USA
[4] Ctr Dis Control & Prevent, Biotechnol Core Facil Branch, Atlanta, GA USA