1 National Centre for Immunisation Research and Surveillance, Children's Hospital at Westmead, Westmead, NSW 2145, Australia; Discipline of Child and Adolescent Health, Sydney Medical School, University of Sydney, Sydney, NSW, Australia; Marie Bashir Institute for Infectious Diseases and Biosecurity, School of Biological Sciences and Sydney Medical School, University of Sydney, Sydney, Australia; WHO Collaborating Centre for Mass Gatherings and High Consequence/High Visibility Events, Flinders University, Adelaide, SA, Australia. Electronic address: robert.booy@health.nsw.gov.au.
2 Discipline of Child and Adolescent Health, Sydney Medical School, University of Sydney, Sydney, NSW, Australia; Marie Bashir Institute for Infectious Diseases and Biosecurity, School of Biological Sciences and Sydney Medical School, University of Sydney, Sydney, Australia.