Probabilistic linkage of national immunisation and state-based health records for a cohort of 1.9 million births to evaluate Australia’s childhood immunisation program

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Heather F Gidding Lisa McCallum Parveen Fathima Thomas Snelling Bette Liu Nicholas de Klerk Christopher C Blyth Vicky Sheppeard Ross M Andrews Louisa Jorm Peter B McIntyre Hannah C Moore
Published online: Sep 18, 2017

Several countries have developed national immunisation registers, but only the Nordic countries
have linked their registers to other health data in order to comprehensively evaluate the ‘real world’
effectiveness of vaccines. Nordic countries can link datasets deterministically using the national
person identifier, but most countries, including Australia, don’t have such an identifier to enable
this type of linkage.

To describe the process for assembling a linked study cohort that will enable the conduct of
population-based studies related to immunisation and immunisation policy.

National death and immunisation databases along with state health data (notifications of vaccine
preventable diseases, perinatal data, hospital admissions and emergency department presentations)
up until December 2013 were probabilistically linked (using demographic details) for children born
between 1996 and 2012 in two states: Western Australia and New South Wales (42% of Australia’s
population, combined).

After exclusions there were 1.95 million children in the study cohort (live born children with
both a birth and perinatal record which represents 97.5% of all live births in the state perinatal
data collections - our source population) and 18.0 million person years of follow up (mean: 9.2
years per child). The characteristics of children in the cohort were generally similar to those only
included in state perinatal databases and outcome measures were in keeping with expected figures
from unlinked data sources. However, the lack of a dynamic national population register meant
immigrants could not be included.

We have been able to develop a similarly comprehensive system to the Nordic countries based on
probabilistic linkage methods. Our experience should provide encouragement to other countries
with national immunisation registers looking to establish similar systems.

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