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

Pertussis

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

Pertussis (whooping cough) is an acute illness, caused by the Bordetella pertussis bacterium, involving the respiratory tract. Typically illness begins with an irritating cough that gradually becomes paroxysmal and lasts for one to two months or longer. Paroxysms are characterised by repeated violent coughs and are followed by a characteristic crowing or high-pitched inspiratory whoop. Infants less than 6 months of age, adolescents and adults often have fewer classical symptoms without paroxysms or whoop.36

Case definitions


Notifications

National definition from January 2004:10

Confirmed cases are those with definitive laboratory evidence, laboratory suggestive evidence and clinical evidence, or clinical evidence and an established epidemiological link to a confirmed case. Probable cases require clinical evidence (as below) only and are also notified.

Laboratory definitive evidence

Isolation of Bordetella pertussis from a clinical specimen or detection of B. pertussis by nucleic acid testing.

Laboratory suggestive evidence

Seroconversion or significant increase in antibody levels or fourfold or greater rise in titre to B. pertussis, in absence of recent vaccination; or

Single high IgA titre to whole cells; or

Detection of B. pertussis antigen by immunofluorescence assay (IFA).

Clinical evidence

A coughing illness lasting two or more weeks; or

Paroxysms of coughing or inspiratory whoop or post-tussive vomiting.

(See Appendix D for pre-2004 definition)

Hospitalisations and deaths

The ICD-10-AM/ICD-10 code A37 (whooping cough) was used to identify hospitalisations and deaths.

 Distribution by Indigenous status and age

Of the total 29,050 notifications of pertussis recorded in New South Wales, the Northern Territory, South Australia, Victoria and Western Australia over the four years from 2003 to 2006, 439 (1.5%) were identified as occurring in Aboriginal and Torres Strait Islander people (Table 11). For hospitalisations, 111 (11%) of the total 1,057 cases were identified as Aboriginal and Torres Strait Islander in the three-year period July 2002 to June 2005 in New South Wales, the Northern Territory, Queensland, South Australia and Western Australia (Table 12).

Table 11. Pertussis notification rates, selected Australian states, 2003 to 2006, by age group and Indigenous status

Age group
(years)
Indigenous status
Notifications*
(2003–2006)
n Rate Rate ratio
0–4 Indigenous
105
64.6
1.6||
Other
1,532
41.7
5–14 Indigenous
70
21.5
0.6||
Other
2,946
37.4
15–24 Indigenous
56
21.0
0.5||
Other
3,245
39.1
25–49 Indigenous
127
29.5
0.6||
Other
10,589
47.9
50+ Indigenous
81
54.9
1.0
Other
10,293
54.3
All ages Indigenous
439
37.2
0.8
Other
28,611
46.9

* Notifications (New South Wales, the Northern Territory, South Australia, Victoria and Western Australia only) where the date of diagnosis was between 1 January 2003 and 31 December 2006.

† Average annual age-specific rate per 100,000 population.

‡ Rates for all ages combined are age-standardised to the Australian Bureau of Statistics Australian population estimates for 2005.

|| Shaded cells indicate statistically significant, 95% confidence intervals greater or less than 1 (p<0.5).

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Table 12. Pertussis hospitalisations and deaths, selected Australian states, 2002 to 2005, by age group and Indigenous status

Age group
(years)
Indigenous status
Hospitalisations*
(July 2002–June 2005)
Deaths
2003–2005
n Rate Rate ratio n
0–4 Indigenous
93
57.0
3.0||
0
Other
479
18.7
0
5–14 Indigenous
5
1.5
1.0
0
Other
82
1.5
0
15–24 Indigenous
1
0.4
0.9
0
Other
24
0.4
0
25–49 Indigenous
9
2.1
2.2
0
Other
149
1.0
0
50+ Indigenous
3
2.1
1.3
0
Other
212
1.6
0
All ages§ Indigenous
111
5.2
2.3||
0
Other
946
2.3
0

* Hospitalisations (New South Wales, the Northern Territory, Queensland, South Australia and Western Australia only) where the date of separation was between 1 July 2002 and 30 June 2005.

† Deaths (the Northern Territory, Queensland, South Australia and Western Australia only) where the death was recorded between 1 January 2003 and 31 December 2005.

‡ Average annual age-specific rate per 100,000 population.

§ Includes cases with unknown ages. Rates for all ages combined are age-standardised to the Australian Bureau of Statistics Australian population estimates for 2005.

|| Shaded cells indicate statistically significant, 95% confidence intervals greater than 1 (p<0.5).

Both hospitalisation and notification rates were highest among children 0–4 years of age identified as Aboriginal and Torres Strait Islander (57 and 65 per 100,000, respectively). The rates were much lower in older age groups, the difference being more marked in hospitalisations than notifications. While hospitalisation rates were below 3 per 100,000 in older age groups, notification rates in Aboriginal and Torres Strait Islander people were somewhat lower in adults aged less than 50 years, but almost as high in those aged 50 years or more (55 per 100,000) as in young children (Figure 10).

Figure 10. Pertussis notification rates, selected Australian states,* 2003 to 2006, by age group and Indigenous status

Figure 10. Pertussis notification rates, selected Australian states, 2003 to 2006, by age group and Indigenous status

* New South Wales, the Northern Territory, South Australia, Victoria and Western Australia.

† Notifications where the date of diagnosis was between 1 January 2003 and 31 December 2006.

The Indigenous to non-Indigenous rate ratios for notifications provided a very different picture of differential disease burden to that of hospitalisations (Table 11 and Table 12). For notifications, the rate ratio was significantly above 1.0 in the 0–4 year age group but significantly below 1.0 in most other age groups (Table 11). However, for hospitalisations, the rate ratios were above 1.0 in all age groups, except the 15–24 year age group, and these were statistically significant in the 0–4 year age group and all ages combined (Table 12).

During the period 2003 to 2005, there were no reported deaths due to pertussis in the Northern Territory, Queensland, South Australia and Western Australia.

Over the six-year period July 1999 to June 2005, there was substantial fluctuation in the age-adjusted hospitalisation rates for pertussis in Indigenous and non-Indigenous people. In particular, two peaks reflected widespread epidemics in 2001 and 2005 (Figure 11). The hospitalisation rate for non-Indigenous people was consistently below that for Indigenous people, and this difference was statistically significant in most years. During the peak in Indigenous cases in 2001/2002, there was a notable increase in the Northern Territory and Queensland, while the peak in 2004/2005 was related to more Indigenous cases in Western Australia (data not shown).

Figure 11. Pertussis hospitalisation rates, selected Australian states,* 1999 to 2005, by Indigenous status

Figure 11. Pertussis hospitalisation rates, selected Australian states, 1999 to 2005, by Indigenous status

* The Northern Territory, Queensland, South Australia and Western Australia.

† Hospitalisations where the date of separation was between 1 July 1999 and 30 June 2005. Rates are age-standardised to the Australian Bureau of Statistics Australian population estimates for 2005.

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Comment

Universal childhood pertussis vaccination has been in place since the 1950s in Australia and an adolescent booster since 2004, earlier in some jurisdictions.5

Pertussis is the least well controlled of all diseases with long-standing, well-established vaccination programs. It has the highest notification rate for all ages for the total Australian population, and higher hospitalisation rates than most other vaccine preventable diseases. Interpretation of the data is complex, as the epidemiology of the disease has changed with waning immunity and changes in vaccination policy, and notification and hospitalisation data reflect different patterns of disease severity, and also perhaps the effects of changing patterns of diagnostic testing.5 However, some conclusions can be drawn from these data.

Figure 11 shows that epidemics continue to occur in Aboriginal and Torres Strait Islander Australians, varying in time by geographic area. The disparity in hospitalisations has been linked to delayed vaccination in Aboriginal and Torres Strait Islander infants and environmental living conditions.71 In other age groups, the situation is less clear, as the lower notification rates in Indigenous compared with non-Indigenous people in older age groups may reflect lower morbidity and/or poorer access to diagnostic testing for milder disease.

The relatively recent introduction of adolescent vaccination is expected to be reflected in reduced notifications in the near future and its impact on the disease disparity in Aboriginal and Torres Strait Islander infants should be monitored.

Pertussis continues to circulate, causing periodic epidemics in adolescents and adults and transmission to infants who are most vulnerable to severe disease. There is a disproportionate impact on Aboriginal and Torres Strait Islander infants. This underlines the importance of timely vaccination of infants.