The Australian Childhood Immunisation Register: validation of the immunisation status of children who are very overdue

OBJECTIVES: To determine the true immunisation status of children identified by the Australian Childhood Immunisation Register (ACIR) as 90 days overdue- and determine why appropriately immunised children were flagged as overdue. METHODS: A telephone survey of immunisation providers and/or parents of a stratified random sample of 850 NSW children born on or after 1 January 1996 and identified by the ACIR as 90 days overdue for at least one scheduled immunisation at 17 June 1997. The survey was conducted in June to September 1997. RESULTS: Children in the sample ranged in age from 5 to 17 months. Only 526 (61.9%) could be traced. Of these- 452 (86.6%) were fully immunised- and 75% of immunisations were given on time (within 30 days of falling due). The overall proportion of NSW children identified by ACIR as 90 days overdue who were fully immunised was an estimated 85% (95% CI 82.6%-87.4%). For the 452 fully immunised children- a reason for the child's immunisation not appearing on the register could be attributed for only 248/452 (54.8%). There was evidence that the provider had failed to submit an encounter form for 141 of these children. CONCLUSIONS: In mid-1997- more than half the children identified by ACIR as 'overdue' were fully immunised. A significant reason for fully immunised continuing to be flagged as overdue was failure to return encounter forms. IMPLICATIONS: At the time of survey the ACIR could not accurately identify unimmunised children and was of limited use as a tool for public health services to follow up very overdue children.