1. From the Department of Infectious Disease Epidemiology, Imperial College London, London (I.M.B., M.P.-S., N.A.M., N.C.G.); the Polio Eradication Department, World Health Organization, Geneva (O.M.D., P.C., M.Z., R.W.S.); and the Bill and Melinda Gates Foundation, Seattle (A.S.B.).