1. Vaccine Uptake Group, Murdoch Children’s Research Institute, Parkville 3052, Australia
2. The National Child Health Poll, The Royal Children’s Hospital, Parkville 3052, Australia
3. Clinical Epidemiology and Biostatistics Unit, Murdoch Children’s Research Institute, Melbourne 3052, Australia
4. Department of General Medicine, The Royal Children’s Hospital, Parkville 3052, Australia
5. Department of Paediatrics, The University of Melbourne, Parkville 3052, Australia