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

Influenza and pneumonia

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

Influenza is an acute respiratory illness caused by influenza type A or B viruses. Symptoms include abrupt onset of fever, cough, malaise, myalgia, sore throat, and headache. Influenza epidemics usually occur during the winter months in temperate climates, causing an increase in hospitalisations for pneumonia and exacerbation of chronic diseases, and also contributing to increased mortality, particularly among the elderly and those with high risk underlying conditions. The most common complication of influenza is pneumonia. It is generally believed that hospitalisations and deaths coded as influenza significantly underestimate disease burden, with excess all-cause pneumonia and influenza combined, during the influenza season, being a better indicator of true burden.55

Case definitions


Notifications

Not included, due to low completeness of Indigenous status.

Hospitalisations and deaths

The ICD-10-AM/ICD-10 codes J10 and J11 (influenza) were used to identify hospitalisations and deaths from influenza. The ICD-10-AM/ICD-10 codes J10 to J18 were used to identify hospitalisations and deaths from influenza and all-cause pneumonia combined.

 Distribution by Indigenous status and age

Of the total 7,378 hospitalisations for influenza and 223,863 hospitalisations for influenza and pneumonia combined in New South Wales, the Northern Territory, Queensland, South Australia and Western Australia in the three-year period July 2002 to June 2005, 566 (8%) and 16,680 (7%) were identified as occurring in Aboriginal and Torres Strait Islander people, respectively (Table 6).

Table 6. Influenza and pneumonia hospitalisations and deaths, selected Australian states, 2002 to 2005, by age group and Indigenous status

Age groups
(years)
Indigenous status
Hospitalisations*
(July 2002–June 2005)
Deaths
2003–2005
Influenza Influenza and pneumonia Influenza Influenza and pneumonia
n Rate Rate ratio n Rate Rate ratio n n Rate Rate ratio
0–4 Indigenous
330
202
2.1||
4,615
2,830
3.5||
0
0
0
0
Other
2,432
95
20,818
813
1
4
0.3
5–14 Indigenous
32
10
0.9
1,083
331
2.2||
0
1
0.5
2.7
Other
587
11
8,165
148
1
5
0.2
15–24 Indigenous
47
18
2.2||
993
380
4.4||
0
2
1.1
8.7||
Other
482
8
4,999
87
1
4
0.1
25–49 Indigenous
91
21
2.6||
5,511
1,301
8.4||
1
46
15.9
19.7||
Other
1,276
8
23,672
155
5
65
0.8
50+ Indigenous
66
46
3.0||
4,477
3,135
2.7||
0
60
64.1
1.1
Other
2,035
16
149,529
1,143
39
3,820
55.8
All ages§ Indigenous
566
38
2.3||
16,680
1,696
3.5||
2
126
25.1
1.5||
Other
6,812
16
207,183
486
47
3,914
17.1

* 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).

The annual age-standardised influenza hospitalisation rate was 38 per 100,000 for Aboriginal and Torres Strait Islander people compared with 16 per 100,000 for people presumed to be non-Indigenous. The equivalent rates for influenza and pneumonia combined were 1,696 and 486, respectively.

The rates for influenza and pneumonia combined are substantially higher than for influenza alone. However, the distribution by age and Indigenous status are broadly similar (see Figure 4 and Figure 5, allowing for different scales). This pattern consists of high rates for both Indigenous and non-Indigenous children 0–4 years of age, falling substantially for the age groups 5–14 and 15–24 years, before rising again in adults aged 25 years or more.

Figure 4. Influenza hospitalisation rates, selected Australian states,* 2002 to 2005, by age group and Indigenous status



Figure 4. Influenza hospitalisation rates, selected Australian states, 2002 to 2005, by age group and Indigenous status

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

† Hospitalisations where the date of separation was between 1 July 2002 and 30 June 2005.

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Figure 5. Influenza and all pneumonia combined hospitalisation rates, selected Australian states,* 2002 to 2005, by age group and Indigenous status



Figure 5. Influenza and all pneumonia combined hospitalisation rates, selected Australian states, 2002 to 2005, by age group and Indigenous status

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

† Hospitalisations where the date of separation was between 1 July 2002 and 30 June 2005.

The overall Indigenous to non-Indigenous rate ratio was 2.3:1 for influenza and 3.5:1 for influenza and pneumonia combined, and both ratios were statistically significantly above 1.0 (Table 6). These rate ratios were greater than 1.0 and statistically significant in all age groups except 5–14 years for influenza. The highest rate ratio was 8 in the 25–49 year age group for influenza and pneumonia combined.

Of the total 49 deaths from influenza and 4,040 deaths from influenza or pneumonia in the Northern Territory, Queensland, South Australia and Western Australia between 2003 and 2005, 2 and 126, respectively, were identified as Aboriginal and Torres Strait Islander. The rate ratio for deaths was highest in those aged 25–49 years (20) and 15–24 years (9).

Over the six-year period July 1999 to June 2005, the age-standardised influenza hospitalisation rates for Aboriginal and Torres Strait Islander people have declined substantially, but still remain higher than in non-Indigenous people (Figure 6). In non-Indigenous people, the rates have also declined, with the exception of the year 2003/2004, but less dramatically. The decline in rates in Aboriginal and Torres Strait Islander people from 1999/2000 occurred predominantly in Western Australia and in adults aged 25 years or more, while increases in 2003/2004 occurred predominantly in 0–4 year olds and in the Northern Territory, South Australia and Western Australia (data not shown). This pattern is not repeated for the combined influenza and pneumonia rates. For the same six-year period, influenza and pneumonia hospitalisation rates in Aboriginal and Torres Strait Islander people have remained substantially higher than the rates for non-Indigenous people, with no evidence that the gap has narrowed (Figure 7). Rates for influenza and pneumonia combined have been relatively stable in both Indigenous and non-Indigenous people, with the exception of a transitory decline in Aboriginal and Torres Strait Islander people in 2001/2002.

Figure 6. Influenza hospitalisation rates, selected Australian states,* 1999 to 2005, by Indigenous status



Figure 6. Influenza 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.

Figure 7. Influenza and pneumonia hospitalisation rates, selected Australian states,* 1999 to 2005, by Indigenous status



Figure 7. Influenza and pneumonia 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

The relatively high morbidity from influenza and related conditions in older Aboriginal and Torres Strait Islander adults led to a specific program for influenza vaccine being funded nationally from 1999 for Aboriginal and Torres Strait Islander adults aged 50 years or more and those aged 15–49 years with risk factors.56 This is in contrast to the non-Indigenous population, for whom influenza vaccine was funded only from age 65 years. From 2007, annual influenza vaccination is recommended for all Aboriginal and Torres Strait Islander adults.57

Influenza hospitalisation time trends are usually dominated by peaks due to the sporadic appearance of new influenza strains. The 2003/2004 peak in influenza hospitalisations reflects a more severe influenza season nationally in 2003, followed by a mild season in 2004.5 However, the disproportionate decline in Aboriginal and Torres Strait Islander hospitalisations from 1999/2000, predominantly in adults, suggests some possible impact of the Indigenous adult vaccination program. This does not appear to be the case for hospitalisations due to pneumonia or influenza, but the latter is a non-specific diagnosis with multiple aetiologies.

Young children experience higher morbidity from influenza than any other age group, and hospitalisation rates in those aged less than 5 years were double those in the previous report. This may reflect higher influenza morbidity in the four states presented in this report, and may also be influenced by different circulating strains or diagnostic practices. The importance of young children, both in terms of their own high morbidity from influenza and their role in transmission of influenza to adults, has been increasingly appreciated internationally in recent years.58 This has led to recommendations in the United States and Canada that all children between 6 months and 5 years or 23 months of age, respectively, receive influenza vaccine.59,60

This recommendation is particularly pertinent to Aboriginal and Torres Strait Islander children in Australia, where influenza may contribute significantly to overall respiratory morbidity in children and in adults they are in contact with, especially in crowded living conditions.

Aboriginal and Torres Strait Islander people continue to experience significantly higher rates of influenza and pneumonia than non-Indigenous people, with children less than 5 years of age and adults aged 25 years or more most at risk.