Department of Statistics, University of Washington, Seattle, WA 98195, USA, Institut de recherches cliniques de Montréal and Université de Montréal, Montréal, Quebec, Canada H2W 1R7, Faculty of Medicine, Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Quebec, Canada H3A 1A2 and Vaccine and Infections Diseases Division, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA.