1. Center for Clinical Epidemiology and Population Health, Marshfield Clinic Research Institute , Marshfield, Wisconsin , USA
2. Coronavirus and Other Respiratory Viruses Division, Centers for Disease Control and Prevention , Atlanta, Georgia , USA
3. Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin , Madison, Wisconsin , USA
4. Influenza Division, Centers for Disease Control and Prevention , Atlanta, Georgia , USA