[1] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Epidemiol, 615 North Wolfe St, Baltimore, MD 21205 USA
[2] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Int Hlth, 615 North Wolfe St, Baltimore, MD 21205 USA
[3] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Mol Microbiol & Immunol, 615 North Wolfe St, Baltimore, MD 21205 USA
[4] Stanford Univ, Dept Pediat, Stanford, CA USA
[5] Stanford Univ, Dept Microbiol & Immunol, Stanford, CA USA