作者单位:Ctr Dis Control & Prevent, Atlanta, GA USA[1]Univ Michigan, Dept Hlth Management & Policy, 1415 Washington Hts,Room M3515, Ann Arbor, MI 48109 USA[2]Univ Michigan, Dept Internal Med, Ann Arbor, MI 48109 USA[3]Univ Michigan, Dept Biostat, Ann Arbor, MI 48109 USA[4]