Susan Lister,¹ Peter B McIntyre,¹ Margaret A Burgess,¹ Eddie D O'Brien²

The Australian Childhood Immunisation Register (ACIR) commenced operation in January 1996 and provides a comprehensive database of children's immunisations in Australia. The ACIR enables implementation of an immunisation recall and reminder system and improved surveillance and reporting of immunisation coverage. Before the introduction of the ACIR, the methods used in assessing coverage varied widely in design and quality, with few studies measuring coverage at national or state-wide level. This is a systematic review of the scope and reliability of estimates of immunisation coverage available in Australia from 1990 to 1998. A total of 108 studies were identified of which 51 were classified as higher quality based on a range of criteria including whether they had a response rate of 50% or better. Commun Dis Intell 1999;23:145-170.

Summary

Introduction

Accurate information on the proportion of children immunised in Australia is essential for the planning of effective immunisation programs. Before the introduction of the Australian Childhood Immunisation Register (ACIR) in January 1996, the methods used in assessing coverage varied widely in design and quality, with few studies measuring coverage at national or state-wide level.

This systematic review of the scope and reliability of estimates of immunisation coverage from 1990 to 1998 was initiated by the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases (NCIRS) to evaluate the available Australian estimates of immunisation status against data from the ACIR.

Methods

Studies were identified and included in the review, using all available sources, if they examined Australian data and were published or produced between January 1990 and June 1998. Eligible studies were abstracted with a standard proforma including type of publication, sample characteristics, validation and outcome measures. Studies were classified as higher quality using a range of criteria, including if they had response rates of 50% or higher. The term 'fully immunised' was defined as coverage of the full course of vaccines scheduled at the time of each study. Data were also tabulated by individual vaccine and age strata (12-23 months, 24-35 months and 4-6 years) where available.

Results

A total of 108 eligible studies were identified and 51 higher quality studies tabulated by location, design, sample size, response rate, strategy and method of validation (Tables 1-7). Studies investigating immunisation procedures and processes, and letters and editorials that did not report original data on immunisation coverage were excluded.

The most common assessment age was from 24 to 35 months and the proportion of children fully immunised ranged from 51% to 88% (excluding Haemophilus influenzae type b (Hib)). The 1995 Australian Bureau of Statistics (ABS) estimates for 24 to 35 months were lower (51.3%) than most other comparable population-based studies (range 60.3% - 88%). Details of the immunisation status of children by age strata are shown in Tables 8-10.

Immunisation coverage in child-care settings for children aged 0-5 years ranged from 60.3% to 70% (excluding Hib) in studies using provider documentation. Coverage in schools for children aged 4-6 years was higher (range 67% to 89% excluding Hib) but likely to be overestimated. While studies in remote Aboriginal communities suggested coverage was much higher than the general population, studies in less remote areas found much lower levels of coverage in Aboriginal children than in the general population.

Comprehensiveness and validity at a national population level were key criteria for data quality, fulfilled only by the ABS surveys. State-wide population databases using provider-held records to assemble a prospective birth cohort had the next highest validity, followed by cross-sectional studies with appropriate sampling and high response rates. Many studies based on retrospective birth cohorts had low response rates with potential selection bias. These more geographically restricted studies have generally produced higher estimates of immunisation coverage than the ABS survey. The first 12 month cohort from the ACIR gave lower coverage estimates for diphtheria-tetanus-pertussis (DTP) and poliomyelitis (OPV), but higher for Hib than the ABS survey. ACIR data are currently incomplete and should be viewed as minimum estimates, but can detect large changes over time such as has occurred with Hib vaccine coverage.

Conclusions

This review showed that methodology strongly influences the final estimates of coverage and supports the need for a nationally consistent methodology, which would make comparisons much easier. The best national estimate of immunisation status in Australian children prior to the ACIR is the 1995 ABS survey. The ACIR should give increasingly accurate estimates as reporting improves. This can be expected due to current incentive initiatives, but specific surveys of coverage in small populations such as urban Aboriginal communities may still be required. The available data from comparable industrialised countries still indicate suboptimal performance by Australia, with coverage for three doses of pertussis-containing vaccines at around 80% compared with estimates of over 90% in the United Kingdom (UK) and the United States of America (USA).

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Introduction

Accurate information on the proportion of children immunised for each vaccine on the recommended schedule is essential for the planning of effective immunisation programs. In Australia, there have been few studies measuring national or state-wide immunisation coverage and the methods of data collection have varied in quality. The Australian Childhood Immunisation Register (ACIR) was introduced in 1996 in part to provide more consistent and comparable information about immunisation coverage, but all immunisations for children less than 7 years of age will not be included until after 2001.

This systematic review of the scope and reliability of estimates of immunisation coverage in Australia since 1990 was initiated by the NCIRS to provide the background against which coverage estimates from the ACIR could be judged. The available literature was examined with reference to the following research questions:

1. With the exception of the ACIR:
   1. What is the best overall estimate of the current immunisation status of Australian children?
   2. (b) What is the best estimate of immunisation status for each scheduled vaccine by age?
2. How do the estimates from other studies compare with those from the ACIR?
3. How does the immunisation status of Australian children compare with overseas estimates?

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Background

Australia has had childhood immunisation programs since the 1920s, with enormous advances in eliminating or reducing the impact of vaccine preventable diseases (VPDs) such as poliomyelitis, tetanus and diphtheria.¹ However, outbreaks of measles and rubella continued to occur in the 1990's and pertussis is endemic, with nine deaths in infants occurring in 1996-97.^2,3 This disease activity is occurring because immunisation coverage remains below the level of 90%-95% required to interrupt transmission of these highly contagious infections.⁴

Concern about Australia's poor record for immunisation coverage prompted a series of national initiatives, beginning with the National Health and Medical Research Council (NHMRC) convening a panel to review services in 1993. The resulting National Immunisation Strategy set targets for immunisation coverage and control of vaccine preventable diseases (VPDs) and recommended initiatives to achieve these targets.⁴

The recommendations of the National Immunisation Strategy were addressed in several ways. The establishment of the National Notifiable Diseases Surveillance System (NNDSS) in 1991 had already begun the process of national surveillance and reporting of VPDs using common case definitions. From 1996 the ACIR provided both an immunisation recall and reminder system for parents and improved surveillance and reporting of immunisation coverage.

In 1997, the Minister for Health and Family Services initiated the 'Seven Point Plan' to increase the proportion of fully immunised children in Australia. This Plan included monetary incentives, commencing in mid 1998, for parents whose children receive child-care assistance payments and incentives for general practitioners whose practices include a high proportion of fully immunised children. The Plan outlined the measles elimination strategy, a range of educational initiatives, a proposal to introduce uniform school entry legislation relating to immunisation status, and enhancement of research activities which led to the establishment of the NCIRS.

The only national immunisation coverage data, prior to the ACIR, came from national surveys by the Australian Bureau of Statistics (ABS). The most recent survey in 1995 found that 52.1% of children aged 0-6 years were fully immunised for age, excluding Hib. The level of full immunisation for the same age group in the previous ABS survey in 1989-90 was similar (54.1%), although these estimates are not directly comparable due to changes in the standard immunisation schedule and the format of the questionnaire.⁵ The ABS survey showed that coverage levels varied between States and Territories, vaccines, age groups and socio-economic and ethnic groups.⁵

Other than the ABS surveys, data on coverage were state-wide or regional and predominantly from ad hoc surveys. Meaningful comparisons between these studies are difficult because methodology and outcome measures were not uniform. Several States and Territories also developed their own population-based vaccination registers (Australian Capital Territory,⁶ Victoria⁷, Queensland^8,9 and Northern Territory^10,11) to obtain more consistent data for analysis of trends. Comprehensiveness of the data from some of these registers can be questioned due to incomplete reporting by providers and because some children receive their immunisations in both the private and public sectors. Other estimates of coverage have come from a variety of sources including outbreak investigations, serological surveys and field vaccine studies, but this was incidental to their main objective and applied only to particular settings.

This review includes both published and unpublished literature from 1990 to 1998 and focuses on studies in which the primary purpose was to estimate immunisation coverage. These data on childhood immunisation coverage in the 1990s provide the background against which the initiatives begun in 1997 may be compared.

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Methods

Search strategy

Studies were included in the review if they gave Australian data and were published or produced between January 1990 and June 1998. University theses and treatises were included, as were conference abstracts and proceedings although efforts were made to identify resulting publications wherever possible. Publication was defined as a peer-reviewed journal, government bulletin or report in the public domain. All other studies were classified as unpublished. Letters and editorials containing relevant data were also included.

While the review focuses on coverage of the primary immunisation schedule in children aged 6 years and under (the age group used in the ACIR and ABS immunisation surveys) studies were also included for young people up to the end of high school. The eligible vaccines were those recommended for use in children during the study period: diphtheria-tetanus-pertussis (DTP), poliomyelitis (OPV), measles, mumps and rubella (MMR), Hib and hepatitis B.

The following sources were searched for studies and reviews on childhood immunisation status in Australia since 1990:

• Medline from 1990 to 1998 using the search terms 'immunisation', 'immunisation programs', 'immunisation status', 'Australia' (in MeSH) and the text words 'immunisation status or cover(age) or rate(s)' and 'vaccination status or cover(age) or rate(s)' for all headings. The term 'vaccination' was coded under 'immunisation'.
• Published and unpublished departmental reports, studies and newsletters from the States and Territories. These were identified by direct contact with immunisation co-ordinators, members of the Communicable Disease Network of Australia and New Zealand (CDNANZ) and the authors themselves.
• Manual and electronic searching of the Communicable Diseases Intelligence journal from 1990-1998.
• University theses and treatises in public health located by personal contact with authors and supervisors.
• Follow-up of references from key reports and publications.
• Abstracts and conference proceedings from the Public Health Association (PHA) National Immunisation Conferences of 1991, 1993, 1995 and 1996 and other relevant conferences in Australia, for example the NSW Public Health Network Conference. The authors of these studies were contacted and full reports obtained wherever possible.

Criteria for eligibility

The following study designs were included in the review:

• cross-sectional or cohort studies directly measuring immunisation coverage, both in the entire population and in specific settings, such as child care centres and schools;
• state immunisation databases and data from vaccine distribution systems;
• immunisation coverage measured as part of outbreak investigations;
• serological surveys; and
• studies were excluded if they were surveys primarily measuring attitudes, knowledge and behaviour of providers and consumers, studies of vaccine adverse events or studies monitoring the vaccine cold chain.

Data abstraction

After assessment of eligibility, identified studies were abstracted using a standard proforma, which included:

• type of publication (published / unpublished);
• study design (population based / non-population based and including sub-sets such as schools, childcare services);
• sample characteristics - design, size and response rates;
• vaccines included;
• measures of immunisation status; and
• validation of immunisation status.

These data were entered into a Reference Manager database.

Quality assessment

A group of higher quality studies from a range of designs and settings were selected for tabulation and comparison with the ACIR. Studies were classified as higher quality using criteria including study design, study population, response rates, sample size and validity. Generalisability was a particularly important criterion. For example, population-based cross-sectional studies using large sample sizes and with high response rates were rated more highly than retrospective birth cohorts with sample sizes of less than 50% and evidence of selection bias. Moreover, studies with validated coverage data were rated more highly than those relying on parental recall.

The peer-reviewed published studies in this review were given greater weight than the unpublished studies. However, several State Health Departments routinely used data from non-peer reviewed publications to estimate and monitor immunisation coverage in their regions. Peer-reviewed publication was therefore not the only important measure of data quality.
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Age strata for coverage assessment

The National Immunisation Strategy in 1993 identified 2 year old children as the primary group for estimates of full immunisation⁴ and this is reflected in the outcomes of many of the studies in this review. More recently, the NHMRC described immunisation outcomes in terms of 'milestones' set at 6 months, 12 months and 18 months.¹² However, the outcomes from the majority of the studies in the review were not classified according to these categories (other than 12 months) and therefore the assessment ages outlined above were used. The Appendix outlines the 1994 NHMRC immunisation schedule current for the 1995 ABS survey and describes the major changes made since the previous survey in 1989-90. The 1996 immunisation schedule is also outlined in the Appendix, including the immunisation 'milestones'.¹²

The outcome measures from the higher quality studies, including data on full immunisation and for individual vaccines, were grouped into the following age strata for comparability with each other and data from the 1995 ABS survey:

• 12-23 months
• 24-35 months; this was the most common assessment age used in the studies
• 4-6 years; to determine school entry immunisation

Definitions of immunisation coverage

Immunisation coverage is normally expressed as the proportion or prevalence (%) of complete immunisation by particular assessment ages or 'milestones'. Most studies in this review have allowed for a 'grace period' of around 1 to 3 months in assessing coverage, which is up to 6 months for the ACIR cohort.¹³ The definition 'fully immunised' used in this review is receipt of the full course of vaccines scheduled at the time of the study for the assessment age. This definition includes using various outcomes such as 'age appropriately immunised' and 'immunised up to date' but only if separate questions have been asked about each vaccine.¹⁴ The full course of vaccines included are those defined in the standard vaccination schedule at the time of the study (Appendix) but it is noted that a small number of studies in this review have accepted the combined diphtheria and tetanus (CDT) vaccine as a substitute for DTP.^8,15

The ABS immunisation surveys

The ABS has conducted national immunisation surveys on a regular basis since 1983, which until the advent of the ACIR, provided the only Australia-wide population, based data on immunisation status. The ABS surveys have been considered a reference standard for estimating immunisation coverage because of their high quality sampling methods.

The most recent survey in 1995 was conducted as part of the regular monthly Labour Force survey, which derives a probability population sample using a stratified, multistage and clustered design.⁵ Each State or Territory was divided into strata, and sampling of Census Collection Districts then undertaken. Around 30,000 private dwellings in total were included, with interviews conducted by trained interviewers over a two week period in April 1995. High response rates, complete population ascertainment and large sample size are notable features of this survey.

Information on children's immunisation and health screening was obtained by parental report for 6,768 children aged 0-6 years from approximately 5,000 households. Of these children, 870 were aged under 1 year, 960 were 12-23 months, 1,021 were 24-35 months, 907 were 36-47 months and 3,010 were aged 4-6 years. Parent Held Records (PHRs) were consulted for 60.6% of children aged 3 months to 6 years. Just over half (52.1%) of the 1995 sample were found to be fully immunised (excluding Hib), and 46% were classified as partially immunised as they had not completed the full course of each vaccine. The remaining children had either an unknown immunisation status (1.1%) or were totally unimmunised (0.4%).⁵

Although the same sample selection procedure is used for all the ABS immunisation surveys, comparability between the 1995 and earlier surveys is limited by changes in the NHMRC schedule and the questions asked.^5,16 These changes are summarised in the Appendix.

The Australian Childhood Immunisation Register

The ACIR began on January 1, 1996 as part of the 1993 National Immunisation Strategy.⁴ The aim of the register was to provide more accurate and comprehensive information about immunisation coverage and to be a key component of an initiative to improve the immunisation status of Australian children.¹³ The register is administered by the Health Insurance Commission (HIC), which is responsible for both routine reports and a recall-reminder system. The database holds immunisation details on all children under the age of 7 years who are registered for Medicare (approximately 98% of children by 12 months of age) and also for any notification of immunisation to the ACIR for children not registered with Medicare.

Immunisation information may be transferred to the ACIR by all providers in both the public and private sectors. Under-reporting is estimated to reduce immunisation coverage by approximately 10%,^17,18 especially in States with a higher proportion of general practitioner (GP) providers such as New South Wales (NSW) and Western Australia (WA). Difficulties have also occurred in the transfer of data to the HIC from some regions.¹⁸

Coverage reports from the ACIR are based on 3 month birth cohorts measured at two 'milestones': 12 months (DTP, OPV and Hib vaccines) and 24 months (MMR, DTP, Hib and OPV vaccines).¹³


This article was published in Communicable Diseases Intelligence Volume 23, No 6, 10 June 1999.