Holly Hagan, Hanne Thiede, Sharon G. Hopkins, and Jeffrey S. Duchin are with the Seattle-King County Department of Public Health, Seattle, Wash. Holly Hagan, Hanne Thiede, Noel S. Weiss, Sharon G. Hopkins, and E. Russell Alexander are with the Department of Epidemiology and Jeffrey S. Duchin is with the Division of Allergy and Infectious Diseases, University of Washington Seattle.