[1] Univ Penn, Robert Wood Johnson Fdn, Clin Scholars Program, Philadelphia, PA 19104 USA
[2] Univ Penn, Leonard Davis Inst Hlth Econ, Philadelphia, PA 19104 USA
[3] Ctr Dis Control & Prevent, Ctr Surveillance Epidemiol & Lab Serv, Atlanta, GA USA
[4] Dept Hlth & Human Serv, Washington, DC USA