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

Varicella

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

Varicella (chickenpox) is a highly contagious infection caused by the varicella-zoster virus (VZV). The average incubation period is 14–16 days. The clinical picture includes sudden onset of slight fever, mild constitutional symptoms and a generalised pruritic vesicular rash. About 5% of infections are subclinical. Acute varicella may be complicated by cerebellitis, aseptic meningitis, transverse myelitis, thrombocytopenia and pneumonia.36

In unvaccinated populations, varicella is primarily a childhood illness with more than 90% of the population in temperate countries developing clinical or serological infection by adolescence.86 In Australia, seropositivity was 83% by age 10–14 years in the pre-vaccine era.87 Varicella is generally a benign, self-limiting illness in children, but morbidity and mortality rates are higher in adults,88 at the extremes of ages, and in the immuno­compromised.89

Case definitions


Notifications

Not available for the period covered in this report.

Hospitalisations and deaths

The ICD-10-AM/ICD-10 code B01 (varicella [chickenpox]) was used to identify varicella hospitalisations and deaths.

 Distribution by Indigenous status and age

Of the total 3,205 hospitalisations for varicella in New South Wales, the Northern Territory, Queensland, South Australia and Western Australia over the three-year period July 2002 to June 2005, 215 (7%) were identified as occurring in Aboriginal and Torres Strait Islander people (Table 15).

Table 15. Varicella 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
127
78
1.8||
0
Other
1,088
43
1
5–14 Indigenous
42
13
1.5||
0
Other
469
8
0
15–24 Indigenous
18
7
1.3
0
Other
306
5
0
25–49 Indigenous
26
6
1.2
0
Other
802
5
1
50+ Indigenous
2
1
0.6
0
Other
325
2
2
All ages§ Indigenous
215
10
1.4||
0
Other
2,990
7
4

* 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 incidence was highest in 0–4 year olds for Indigenous and presumed non-Indigenous people (Table 15, Figure 15). This age group represents 59% (127) of all hospitalisations identified as Indigenous and 36% (1,088) of hospitalisations in those presumed to be non-Indigenous.

Figure 15. Varicella hospitalisation rates, selected Australian states,* 2002 to 2005, by age group and Indigenous status

Figure 15. Varicella 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 of 1.4:1 for hospitalisations was statistically significantly above 1.0, and also significantly above 1.0 for the age groups 0–4 and 5–14 years (Table 15).

Of the four deaths recorded from varicella in the Northern Territory, Queensland, South Australia and Western Australia, none were recorded for people identified as Aboriginal and Torres Strait Islander.

Over the six-year period July 1999 to June 2005, hospitalisation rates for varicella were statistically significantly higher for Indigenous people compared with people presumed to be non-Indigenous in years 2002/2003, 2003/2004 and 2004/2005 (Figure 16). For Indigenous people, there was no statistically significant change over time, while, for non-Indigenous people, there was a significant decrease in 2002/2003 followed by an increase in 2003/2004. Increases were consistent across age groups and jurisdictions (data not shown).

Figure 16. Varicella hospitalisation rates, selected Australian states,* 1999 to 2005, by Indigenous status

Figure 16. Varicella 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

Varicella vaccination was recommended from September 2003 for all Australian infants, and adolescents who have no clinical history of chickenpox, and funded for these groups from November 2005.20

Chickenpox is not known as a disease of particular concern for Aboriginal and Torres Strait Islander people compared to others, and there are no known previous publications comparing the disease burden in Indigenous and non-Indigenous people, nationally or internationally. The hospitalisation data presented here show a statistically significantly higher rate in Indigenous compared with non-Indigenous children up to 14 years of age and all ages combined, but the scale of this disparity is smaller than most other vaccine preventable diseases (see Appendix B). As these data were all collected in the pre-vaccination era, and at least 90% of people are infected with varicella during their lifetime, the reasons for the slightly higher hospitalisation rates in Aboriginal and Torres Strait Islander people may be due to poorer access to primary care, particularly in remote areas, or a greater propensity for complicating skin and soft tissue infections, related to environmental living conditions. Changes in hospitalisation rates over time were not limited to particular demographic, age or geographic categories.

Varicella notifications were not reported here, as national surveillance only commenced in 2006, and surveillance was incomplete in that year.20,90

Aboriginal and Torres Strait Islander people suffer slightly higher hospitalisation rates for chickenpox compared with other Australians, and therefore should gain substantial benefits from funded universal vaccination, which commenced in November 2005.