[1] Johns Hopkins Bloomberg Sch Publ Hlth, Dept Epidemiol, Baltimore, MD 21205 USA
[2] Boston Univ, Sch Publ Hlth, Dept Hlth Law Policy & Management, 715 Albany St,Room Talbot 242 West, Boston, MA 20118 USA
[3] Johns Hopkins Sch Med, Div Gen Pediat & Adolescent Med, Baltimore, MD USA