[1] Univ Calif Los Angeles, Sch Publ Hlth, Dept Epidemiol, Los Angeles, CA 90095 USA
[2] Univ Calif Los Angeles, Sch Publ Hlth, Ctr Rapid Influenza Surveillance & Res, Los Angeles, CA 90095 USA
[3] Univ Michigan, Sch Publ Hlth, Dept Epidemiol, Ann Arbor, MI 48109 USA
[4] NIAID, Infect Dis Lab, NIH, Bethesda, MD 20892 USA