Vaccine Preventable Diseases and Vaccination Coverage in Aboriginal and Torres Strait Islander People, Australia, 2003 to 2006

Methods: Vaccine preventable diseases data

Disclaimer: Produced by the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases and the Australian Institute of Health and Welfare on behalf of the Australian Government Department of Health and Ageing. Published as a supplement to the Communicable Diseases Intelligence journal Volume 32, June 2008.

Page last updated: 30 June 2008

Three sources of routinely collected data were used for this report. Notification data were obtained from the National Notifiable Diseases Surveillance System (NNDSS), hospitalisation data from the Australian Institute of Health and Welfare National Hospital Morbidity Database, and mortality data from the AIHW Mortality Database.

In this report, Indigenous status consists of two categories: ‘Indigenous’ which records whether a person is identified as being of Aboriginal or Torres Strait Islander origin, and a composite category, ‘other’, which includes those recorded as non-Indigenous and those listed as ‘not stated/inadequately described’.2

Notifications

The NNDSS database was established in its current form in 1991, and includes information about cases of vaccine preventable diseases (VPDs) reported by laboratories and health workers to state and territory authorities under their public health legislation.

State and territory notification criteria are based on the National Health and Medical Research Council (NHMRC) surveillance case definitions.9 However, historically, application of these definitions has differed between jurisdictions, with some using the 1994 NHMRC case definitions as written (e.g. South Australia and Western Australia) and others using their own definitions (e.g. New South Wales and Victoria). In September 2003, new national case definitions for notifications reported to NNDSS were endorsed by the Communicable Diseases Network Australia.10 All the jurisdictions included in this report implemented the new definitions in January 2004, except New South Wales which commenced in August 2004.

Disease notifications for cases with a date of diagnosis between 1 January 2003 and 31 December 2006 (four years), as at May 2007, are included in this report. Previous reports analysed notifications by date of onset as collected from the clinical history, where available, or the specimen collection date for laboratory-reported cases. As of mid 2005, a date of diagnosis field was generated for all NNDSS records. Date of diagnosis is determined using an algorithm whereby the earliest date in the fields date of onset, date of specimen, date of notification and date notification received (the only compulsory date field) is selected.11

The variables extracted for analysis for every disease were: date of diagnosis, Indigenous status, age at onset, and the state or territory from which the notification was received. Following an assessment of the completeness of the Indigenous status field (see below), notifications were included for New South Wales, the Northern Territory, South Australia, Victoria and Western Australia.

Detailed notification data are presented for Haemophilus influenzae type b (Hib) disease, hepatitis A, acute hepatitis B, measles, meningococcal disease, pertussis, and pneumococcal disease. For VPDs with few or no notifications in the period (diphtheria, mumps, polio, rubella and tetanus), data are not presented in the results section but summary data are presented in Appendix A. Varicella data are not presented as they are available only for part of the relevant period (2006), and neither rotavirus nor HPV is included as they were not nationally notifiable and a vaccine was not available in the period covered by this report. Data are not provided for influenza notifications due to the low level of completeness of the Indigenous status field.

Hospitalisations

The AIHW National Hospital Morbidity Database has received administrative, demographic and clinical information about patients admitted to public and private hospitals in Australia since 1993. Data are received by financial year of separation (discharge). The three most recent years for which data were available (2002/2003, 2003/2004, 2004/2005) are included in this report. Following AIHW recommendations on Indigenous identification data, only five states are included: New South Wales, the Northern Territory, Queensland, South Australia and Western Australia.2 New South Wales hospitalisations have been included for the first time, following the demonstration of satisfactory rates of recording Indigenous status in hospitalisations in 2004–2005, and comparable reporting rates in the previous three years (Dr Fadwa Al-Yaman, AIHW, personal communication, October 2007). Trends over time for each disease for the years 1999/2000 to 2004/2005 include data from only the four jurisdictions where Indigenous status has been demonstrated as satisfactory over the whole period – the Northern Territory, Queensland, South Australia and Western Australia.

Data were extracted based on the International Statistical Classification of Diseases and Related Health Problems, 10th Revision, Australian Modification (ICD-10-AM). Eligible separations included those with the code of interest listed in the principal diagnosis (the diagnosis chiefly responsible for the admission of the patient to hospital) or in any other diagnosis field. For acute hepatitis B, only principal diagnoses were included, consistent with previous practice.12

The variables extracted for analysis were: age at admission, state or territory of residence, Indigenous status, year of separation, and causes of admission (principal and other diagnoses – up to 31 diagnoses were recorded for each admission).

Detailed hospitalisation data are presented for hepatitis A, acute hepatitis B, influenza and pneumonia, measles, meningococcal disease, pertussis, pneumococcal disease and varicella. Separation data are not presented in the results section for those VPDs with few or no separations during the period (diphtheria, mumps, polio, rubella, tetanus); summary data are presented in Appendix B. No hospitalisation data are presented for invasive Hib disease as no type-specific code exists.

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Deaths

Death data were obtained from the AIHW Mortality Database. These data are supplied annually to the AIHW from the Registrars of Births, Deaths and Marriages in each state and territory via the Australian Bureau of Statistics. Deaths include those in Australian waters as well as on Australian soil, whereas ABS published data exclude deaths in Australian waters. Since 1997, the International Classification of Diseases, 10th Revision (1992) (ICD-10) has been used to identify the cause of death. Although multiple causes of death have been recorded since 1997, this report included only those where the underlying cause of death was recorded as the disease of interest.

Deaths analysed in this report were for the three most recent years for which data were available (2003, 2004, 2005). The variables extracted for each death were: underlying cause, age, year the death was reported, Indigenous status, and state or territory in which the death was recorded.

Following previous practice,3 mortality data were analysed only for those jurisdictions that met criteria for reliable reporting of Aboriginal and Torres Strait Islander deaths (the Northern Territory, Queensland, South Australia and Western Australia). For diseases in the results section, numbers of deaths are presented by age group. For those VPDs not included in the results section (diphtheria, mumps, polio, rubella and tetanus), summary data are provided in Appendix B.

Calculations and statistical methods

Incidence rates in Aboriginal and Torres Strait Islander people were calculated using the low series of ABS experimental estimates and projections, Aboriginal and Torres Strait Islander Australians, 30 June 1991 to 20 June 2009, for the relevant jurisdictions, from the 2001 Census. Incidence rates for other (presumed non-Indigenous) persons were calculated using as the denominator the total ABS-estimated resident population for the relevant jurisdictions as at June of that year, minus the relevant jurisdiction’s Aboriginal and Torres Strait Islander population.

Direct standardisation is used to calculate rates for all ages combined, using the ABS 2005 population estimates. For hospitalisation data, the mid-year population estimate for the first half of the financial year was used as the denominator; for example, the June 2002 population estimate was used to calculate rates for 2002/2003.

A rate ratio for Indigenous versus other persons was calculated for each disease, with age-specific rate ratios where appropriate. All rates are presented as average annual rates per 100,000 total population or population by age group, as appropriate.

The 95% confidence intervals for rates were calculated from the Poisson distribution of the number of cases. For rate ratios, 95% confidence intervals were calculated using the method of Rothman,13 and regarded as statistically significant if confidence intervals did not overlap 1.0.

Confidence intervals for age-standardised rates used the method of Draper.14