1. These authors contributed equally to this work
2. Vaccine & Immunisation Research Group, Melbourne School of Population Health, University of Melbourne and Murdoch Children’s Research Institute, Melbourne, Australia
3. World Health Organization Collaborating Centre for Reference and Research on Influenza, Melbourne, Australia
4. Surveillance Branch, Office of Health Protection, Australian Government Department of Health and Ageing, Canberra, Australia
5. Research and Business Development Division, Australian Red Cross Blood Service, Sydney, Australia