Objective: To assess the level of under-reporting to the Australian Childhood Immunisation Register (ACIR) and the resulting underestimation of national immunisation coverage using ACIR data, and to correct national immunisation estimates for under-reporting.
Methods: A national population-based telephone survey was conducted in May-July 2001 of two random samples of children born in 1998 and 1999 who were recorded on the ACIR as incompletely immunised at either 12 months or 24 months of age. Parents were asked whether and when their child had received the vaccinations required to qualify as fully immunised. Survey data were then used to correct ACIR-derived coverage estimates at 12 and 24 months of age.
Results: Of 640 surveyed children in the 12-month group, 258 (40%) met the study definition of 'definitely immunised'. This adjusted the ACIR coverage estimate upwards by 2.7% to 94% (95% CI 93.6-94.1). Of 698 surveyed children in the 24-month group, 387 (55%) met the study definition of 'definitely immunised' at the second birthday. Adjusted coverage for doses due by 24 months was 89.8% (95% CI 89.6-90.1), 5% higher than recorded on the ACIR.
Conclusions: Immunisation coverage in Australia for all scheduled vaccines due by 12 months of age is 94% and for all vaccines due by two years of age is almost 90%. The ACIR underestimates coverage by up to 5%. As the ACIR database relies on provider notification, published estimates of immunisation coverage are unlikely to rise significantly above current levels, unless mechanisms are put in place to further improve notification to the ACIR.