1. Center for Statistics and Quantitative Infectious Diseases, Vaccine and Infectious Disease Division, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA.
2. Department of Biostatistics, University of Washington, Seattle, WA 98105, USA.
3. Center for Statistics and Quantitative Infectious Diseases, Department of Biostatistics, College of Public Health and Health Professions, and College of Medicine, University of Florida, Gainesville, FL 32611, USA.