1. Department of Epidemiology and Center for Rapid Influenza Surveillance and Research University of California Los Angeles School of Public Health Los Angeles, CA 90095, USA
2. Department of Epidemiology University of Michigan School of Public Health Ann Arbor, MI 48109, USA
3. Laboratory of Infectious Diseases National Institute of Allergy and Infectious Diseases National Institutes of Health Bethesda, MD 20892, USA