Vaccine Preventable Diseases and Vaccination Coverage in Australia, 2003 to 2005

Varicella-zoster virus infection

Disclaimer: This is the fourth report on vaccine preventable disease and vaccination coverage in Australia, and is 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 31, June 2007.

Page last updated: 20 July 2007

Varicella (chickenpox) is a highly contagious infection caused by the varicella-zoster virus (VZV). The average incubation period is 14–15 days, and is followed by the appearance of a rash. About 5% of infections are subclinical. Acute varicella may be complicated by cerebellitis, aseptic meningitis, transverse myelitis, thrombocytopenia and pneumonia.44

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.274 In Australia, however, seropositivity was 83% by age 10–14 years.275 Varicella is generally a benign, self-limiting illness in children, but morbidity and mortality rates are higher in adults,276 at the extremes of ages, and in the immunocompromised.277 A universal infant varicella vaccination program was introduced in Australia in 2005.

Herpes zoster (HZ) or shingles is a sporadic disease, caused by reactivation of latent VZV. It is usually self-limiting and is characterised by severe dermatomal pain, often followed by post-herpetic neuralgia, which can be chronic and debilitating in the elderly.278 Although herpes zoster can occur at any age, most cases occur after the age of 50 and incidence increases with age.279 However, children infected in utero or those who acquire varicella before the age of one year, and patients on immunosuppressive drugs or infected with human immunodeficiency virus, are also at increased risk of herpes zoster.280–282 A new herpes zoster vaccine which is over 60% effective in reducing the burden of herpes zoster and post-herpetic neuralgia283 is likely to be available in Australia in 2007.

Case definitions

Notifications

Varicella is not a nationally notifiable disease. Varicella and herpes zoster became notifiable in South Australia in 2002. Varicella zoster virus infection became laboratory notifiable in Queensland in December 2005. Other jurisdictions are considering varicella zoster surveillance.

Hospitalisations and deaths

The ICD-10-AM/ICD-10 code B01 (varicella [chickenpox]) was used to identify varicella hospitalisations and deaths. The ICD-10-AM/ICD-10 code B02 (zoster [shingles]) was used to identify herpes zoster hospitalisations and deaths.

South Australian surveillance data

South Australian notification data were included in this report. Varicella and herpes zoster have been notifiable diseases in South Australia since 2002. Clinical diagnoses of chickenpox or herpes zoster, and laboratory diagnoses of varicella-zoster virus infection, are considered confirmed cases for the purposes of surveillance.

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Secular trends, varicella and herpes zoster

There were 4,281 hospitalisations (average annual hospitalisation rate 7.2 per 100,000) for varicella between 1 July 2002 and 30 June 2005 (Table 23). A median of 124 cases of varicella (range 60–197) was hospitalised per month (Figure 46). The rate and median monthly admissions are slightly lower than the period 2000–2002, but longer term surveillance is required to determine if this is random fluctuation or a meaningful decrease.

Figure 46 shows that there are significantly more hospitalisations for herpes zoster than varicella. There were 14,926 hospitalisations (average annual hospitalisation rate 25 per 100,000 for all herpes zoster and 10.7 per 100,000 for herpes zoster as a principal diagnosis), between 1 July 2002 and 30 June 2005 (Table 25). A median of 410 cases of herpes zoster (range 310–465) were hospitalised per month (Figure 46). The rate and median monthly admissions are very slightly higher than the period 2000 to 2002, but longer term surveillance is required to determine if this is random fluctuation or a meaningful increase.

There was demonstrable seasonality, with hospitalisations for varicella peaking in late spring/early summer and dropping in late summer/early autumn.

Figure 46. Varicella and herpes zoster hospitalisations, Australia, July 1993 to June 2005,* by month of admission

Figure 46. Varicella  and herpes zoster hospitalisations, Australia, July 1993 to June 2005,  by month of admission

* Hospitalisations where the month of admission was between 1 July 1993 and 30 June 2005.

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Severe morbidity and mortality, varicella

For patients with an ICD-10-AM code for chickenpox, 21,872 hospital bed days (average 7,290 per year) were recorded from 2002–2005. Of the 4,281 varicella hospitalisations, 2,816 (66%) had a principal diagnosis of varicella (average annual rate 4.7 per 100,000) (Table 23). Complications arising from varicella infection were recorded for 1,374 hospitalisations (32%). Of all varicella hospitalisations, 115 (2.7%) were coded as having encephalitis and 433 (10.1%) were coded as having pneumonitis (Table 24). Although most hospitalisations were in the youngest age group, people aged 60 years and over had the longest median length of stay. There were 11 deaths recorded with varicella as the underlying cause in the calendar years 2003–2004, six (54%) of them for people aged 60 years and over. The highest death rate was in children aged 0–4 years.

Table 23. Varicella hospitalisations and deaths, Australia, 2002 to 2005,* by age group

Age group
(years)
Hospitalisations
3 years
(July 2002–June 2005)
LOS per
separation
(days)
Deaths
2 years
(2003–2004)
  n (§) Rate (§) Median (§) n Rate
0–4
1,597
(1,076)
42.1
(28.4)
2.0 (2.0)
3
0.12
5–14
668
(421)
8.2
(5.2)
2.0 (2.0)
0
0.00
15–24
403
(274)
4.9
(3.4)
2.0 (2.0)
0
0.00
25–59
1,258
(885)
4.3
(3.0)
3.0 (3.0)
2
0.01
60+
355
(160)
3.5
(1.6)
9.0 (7.0)
6
0.09
All ages
4,281
(2,816)
7.2
(4.7)
2.0 (2.0)
11
0.03

* Hospitalisations where the month of separation was between 1 July 2002 and 30 June 2005; deaths where the death was recorded in 2003 or 2004.

† LOS = length of stay in hospital.

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

§ Principal diagnosis (hospitalisations).

Table 24. Indicators of severe morbidity* for hospitalised cases of varicella, Australia, 2002 to 2005,* by age group

Age group
(years)
Varicella encephalitis Varicella pneumonitis
n % of cases n % of cases
0–4
22
1.4
51
3.2
5–14
33
4.9
15
2.2
15–24
10
2.5
41
10.2
25–59
24
1.9
295
23.4
60+
26
7.3
31
8.7
All ages
115
2.7
433
10.1

* Measured using National Hospital Morbidity data where the month of separation was between 1 July 2002 and 30 June 2005.

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Age and sex distribution, varicella

The highest number and rate of varicella hospitalisations occurred in the youngest age groups, especially the 0–4 years age group (Table 23, Figure 47). Within the age group 0–4 years (Figure 48), a decrease in the 12–23 months age group was observed in 2004/2005, a period which includes the introduction of universal varicella immunisation at 12 months of age, compared to the previous years. For the first time, hospitalisation rates were higher in infants aged less than 12 months than in those aged 12–23 months. The overall male:female ratio of hospitalisations was 1.2:1. Males predominated in all age groups except in young adults aged 15–29 years where there was a slight female predominance (data not shown). Of the 11 varicella deaths, five were males.

Figure 47. Varicella hospitalisation rates, Australia, 2002 to 2005,* by age group and sex

Figure 47. Varicella  hospitalisation rates, Australia,  2002 to 2005, by age group and sex

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

Figure 48. Varicella hospitalisation rates, Australia, 2000 to 2005,* by age group (0–4 years) and year of separation

Figure 48. Varicella hospitalisation rates, Australia, 2000 to 2005, by age  group

* Hospitalisations where the date of separation was between 1 July 2000 and 30 June 2005.

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Geographical distribution, varicella

For the years 2002/2003–2004/2005, the Northern Territory had the highest average annual hospitalisation rate (10.6 per 100,000) with all other states recording average annual rates between 6 and 8 per 100,000. Hospitalisation rates in the Australian Capital Territory were lower (average annual rate 3.2 per 100,000) (Appendix 3).

South Australian surveillance data, varicella

Figure 49 shows the notifications of varicella by month of notification from January 2002 to December 2005. There is a clear seasonality in the reported incidence of varicella. A total of 2,564 cases of chickenpox were notified in the current review period (2003–2005), an average annual rate of 56 per 100,000. Figure 50 shows the notifications by gender and age for the current review period. In addition to clinical diagnoses of chickenpox and zoster (reported below), 242 laboratory diagnoses of varicella-zoster were received.

Figure 49. Varicella notifications, South Australia, 2002 to 2005,* by month of notification

Figure 49. Varicella  notifications, South Australia,  2002 to 2005, by month of notification

* Notifications where the date of notification was between 1 January 2002 and 31 December 2005.

Figure 50. Varicella notifications, South Australia, 2003 to 2005, by age group and sex

Figure 50. Varicella  notifications, South Australia,  2003 to 2005, by age group and sex

* Notifications where the date of notification was between 1 January 2003 and 31 December 2005.

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Severe morbidity and mortality, herpes zoster

For patients with an ICD-10-AM code for herpes zoster, 175,164 hospital bed days (average 58,388 per year) were recorded. Of the 14,926 herpes zoster hospitalisations, 6,387 (43%) had a principal diagnosis of HZ (average annual rate 10.7 per 100,000) (Table 25). Complications arising from HZ infection were recorded for 7,576 hospitalisations (50.7%). Of all HZ hospitalisations, 139 (0.93%) were coded as having disseminated HZ and 1,817 (12.2%) were coded as having ocular complications (Table 26). By far the greatest number of hospitalisations were in the oldest age group, who also had the longest median length of stay. There were 38 deaths recorded with herpes zoster as the underlying cause in the calendar years 2003–2004, 36 of them for people 60 years and over. The highest death rate was also recorded in people 60 years and over.

Table 25. Herpes zoster hospitalisations and deaths, Australia, 2002 to 2005,* by age group

Age group
(years)
Hospitalisations
3 years
(July 2002–June 2005)
LOS per separation
(days)
Deaths
2 years
(2003–2004)
n (§) Rate (§) Median (§) n Rate
0–4
108
(81)
2.8
(2.1)
3.0 (3.0)
0
0.00
5–14
269
(183)
3.3
(2.3)
3.0 (3.0)
0
0.00
15–24
224
(120)
2.7
(1.5)
3.0 (3.0)
1
0.02
25–59
2,762
(1,240)
9.5
(4.2)
4.0 (3.0)
1
0.01
60+
11,562
(4,763)
112.6
(46.4)
7.0 (5.0)
36
0.52
All ages||
14,926
(6,387)
25
(10.7)
6.0 (4.0)
38
0.10

* Hospitalisations where the month of separation was between 1 July 2002 and 30 June 2005; deaths where the death was recorded in 2003 or 2004.

† LOS = length of stay in hospital.

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

§ Principal diagnosis (hospitalisations).

|| Includes cases with unknown ages.

Table 26. Indicators of severe morbidity* for hospitalised cases of herpes zoster, Australia, 2002 to 2005,* by age group

Age group
(years)
Zoster encephalitis or meningitis Zoster with other nervous system involvement Disseminated zoster Ocular complications of herpes zoster
n % of cases n % of cases n % of cases n % of cases
0–4
4
3.7
5
4.6
1
0.9
27
25.0
5–14
9
3.3
20
7.4
1
0.4
42
15.6
15–24
13
5.8
20
8.9
4
1.8
29
12.9
25–59
125
4.5
507
18.4
44
1.6
346
12.5
60+
145
1.3
3,744
32.4
89
0.8
1,373
11.9
All ages
296
2.0
4,296
28.8
139
0.9
1,817
12.2

* Measured using National Hospital Morbidity data where the month of separation was between 1 July 2002 and 30 June 2005.

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Age and sex distribution, herpes zoster

The highest number and rate of herpes zoster hospitalisations occurred in the oldest age groups, especially in the over 60 years age group, where the rate was over 112 per 100,000 (Table 25). Across all ages, the male:female rate ratio of hospitalisations was 0.71:1. The male:female rate ratio for deaths due to herpes zoster was 0.53:1.

Geographical distribution, herpes zoster

For the years 2002/2003–2004/2005, South Australia had the highest crude average annual hospitalisation rate for herpes zoster (30.6 per 100,000), followed by Tasmania (29.0 per 100,000). The Northern Territory and the Australian Capital Territory had the lowest rates at around 15 per 100,000, with the other states having rates close to 25 per 100,000 (Appendix 3). That the Northern Territory has the lowest hospitalisation rates for herpes zoster and the highest for varicella presumably reflects the younger age structure of the population of the Northern Territory.

South Australian surveillance data, herpes zoster

Figure 51 shows the notifications of herpes zoster by month from January 2002 to December 2005. A total of 1,751 cases were notified in the current review period (2003–2005), an average annual rate of 38.4 per 100,000. Figure 52 shows the notifications by gender and age for the current review period.

Figure 51. Herpes zoster notifications, South Australia, 2002 to 2005, by month of notification

Figure 51. Herpes  zoster notifications, South Australia,  2002 to 2005, by month of notification

* Notifications where the date of notification was between 1 January 2002 and 31 December 2005.

Figure 52. Herpes zoster notifications, South Australia, 2003 to 2005, by age group and sex

Figure 52. Herpes  zoster notifications, South Australia,  2003 to 2005, by age group and sex

* Notifications where the date of notification was between 1 January 2003 and 31 December 2005.

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Comment

Hospitalisations for herpes zoster are more common than for varicella, even if only the principal diagnosis is considered. In addition, the average length of stay for herpes zoster is four days longer than for varicella, so that the burden of disease caused by severe herpes zoster, as indicated in these hospitalisation data, is greater than that caused by severe varicella.

For varicella, the very young were most commonly hospitalised, while the elderly had the longest length of stay. In our data, 32% of hospitalised cases had a recorded complication. For herpes zoster, just over half of the cases had a complication. This routine data is in agreement with a more detailed study that found that over 50% of herpes zoster hospital episodes had a documented complication, the majority of which were neurological.284 In that study, 16% had ophthalmic zoster, which is a serious complication because it threatens vision.284

Varicella vaccine is included in the routine childhood vaccination schedule in Canada and the USA. In regions of the USA where an active immunisation program for varicella is delivered and there is active disease surveillance, the incidence of varicella has declined dramatically. This is evident in all age groups, and is most marked among those aged 1–4 years.285 Universal VZV vaccination was recommended at 18 months of age in Australia in September 2003, and implemented as a national program in 2005, making it important to have a good understanding of the local epidemiology of disease at baseline. Our early data suggests an impact of the program in 2005 in the age group 12–24 months, with hospitalisations in this group falling below those in the under one year age group for the first time, but longer term trends will need to be observed to confirm this.

In 1952, Hope-Simpson proposed the hypothesis that exposure to varicella may boost immunity against HZ.286 This question has not been addressed in research studies again until recently, when its importance in relation to universal varicella vaccination has become apparent. If exposure to wild varicella provides boosting and protection against activation of HZ, universal infant varicella vaccination and the subsequent decline in wild varicella may result in an increase in HZ incidence.287 There is increasing evidence that exposure to wild VZV does boost immunity to HZ, with two recent observational studies showing lower rates of HZ in groups who are exposed to varicella.288,289 Mathematical modelling suggests that widespread infant VZV vaccination might result in a significant increase in the incidence of HZ, affecting more than 50% of people aged 10–50 years at the time of the introduction of vaccination, with the increase in HZ predicted to persist for over 40 years.287 These predictions might not be correct, particularly if vaccine efficacy is less than that suggested by clinical trial data. Currently surveillance data from the USA, where varicella immunisation has been recommended for over a decade, indicates a large reduction in varicella morbidity with no increase in zoster disease yet demonstrated.290,291

The South Australian notification data show no apparent impact on the transmission of varicella by the availability of VZV vaccine on the private market with a slight increase in reported cases noted per year, although this may well be due to increasing awareness amongst clinicians of the need to notify cases rather than any real change in disease prevalence. Interestingly, the South Australian surveillance system, in contrast to hospitalisations, detects varicella more frequently than herpes zoster. The gender-specific and age-specific data from South Australia show a similar epidemiology to the hospitalised cases, suggesting that hospitalisation data provides a useful measure of trends in varicella and herpes zoster.

Varicella-zoster surveillance has been funded as part of the national varicella immunisation program292 and will include notification to the NNDSS (in five jurisdictions surveillance will comprise passive notification through GPs and labs; in two it will include passive notification as well as syndromic surveillance, with specimen collection through sentinel GPs/emergency departments; and in one state varicella will not be notifiable, with emergency department syndromic surveillance only).293 Additionally trends in disease burden will continue to be reviewed through hospitalisation data and Australian Paediatric Surveillance Unit data on neonatal and congenital varicella infections.

Acknowledgement: Dr Rod Givney, Communicable Diseases Control Branch, Department of Human Services, South Australia.

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