Margaret E Hellard1, Joseph S Doyle2. 1. Centre for Population Health, Burnet Institute, Melbourne, VIC 3004, Australia; Department of Infectious Diseases, The Alfred Hospital, Melbourne, VIC, Australia; Department of Epidemiology and Preventative Medicine, Monash University, VIC, Australia. Electronic address: hellard@burnet.edu.au. 2. Centre for Population Health, Burnet Institute, Melbourne, VIC 3004, Australia; Department of Infectious Diseases, The Alfred Hospital, Melbourne, VIC, Australia; Department of Epidemiology and Preventative Medicine, Monash University, VIC, Australia.