Immunisation coverage reporting through the Australian Childhood Immunisation Register--an evaluation of the third-dose assumption.

OBJECTIVE: The Australian Childhood Immunisation Register (ACIR) currently classifies those children who have the third dose recorded as fully immunised at 12 months of age, even if records of earlier doses are missing. This analysis assesses the impact this "third-dose assumption" has on immunisation coverage estimates for children aged 12 months.
METHODS: ACIR records from three equally spaced cohorts of children at 12 months of age, which relied on the third-dose assumption, were examined for variation in doses and vaccine types recorded by jurisdiction and Medicare registration status.
RESULTS: Although the percentage reduction in coverage without application of the third-dose assumption decreased through the three cohorts examined, the proportion classified as fully immunised still decreased by 11-12% (to < 75%) if the third-dose assumption was not used in the most recent cohort. "Fully immunised" status among children with delayed Medicare registration or in jurisdictions with a high proportion of paper reporting to the ACIR was disproportionately reduced without use of the assumption. CONCLUSIONS AND IMPLICATIONS: While independent sources of data continue to show that the ACIR incorrectly classifies some children as not fully immunised even with the third-dose assumption, its use seems appropriate for reporting population trends in immunisation coverage. Earlier Medicare registration and increased electronic reporting to the ACIR, together with incentives for parents and providers to ensure complete ACIR records, should eventually eliminate the need for the third-dose assumption.