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