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

Haemophilus influenzae type b (Hib) disease

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

Haemophilus influenzae is a Gram-negative bacterium which occurs in both encapsulated and unencapsulated forms. It is a commensal of the nasopharynx, especially in young children. Before Hib vaccines became available, one encapsulated serotype, type b (Hib), caused at least 95% of invasive disease due to H. influenzae in children.59,60 Prior to the introduction of Hib vaccination the most common manifestation of invasive Hib disease was meningitis, with children aged less than 18 months most at risk.59,60 Aboriginal and Torres Strait Islander children had a particularly elevated risk of Hib meningitis, with rates among the highest recorded anywhere in the world, but rarely developed epiglottitis.61 Survivors of Hib meningitis commonly had neurological sequelae such as deafness and intellectual impairment.59,60 Epiglottitis was the other major category of infection, most often occurring in children over the age of 18 months. Other manifestations of Hib disease include cellulitis, septic arthritis, pneumonia, pericarditis, osteomyelitis and septicaemia.

Case definitions

See Appendix 6 for pre 2004 definition

National definition from January 2004:11

Isolation of Haemophilus influenzae type b (Hib) from a normally sterile site where typing has been confirmed at an approved reference laboratory; or

Detection of Hib antigen in cerebrospinal fluid when other laboratory parameters are consistent with meningitis.

Hospitalisations and deaths

There were no ICD-10-AM/ICD-10 codes which specified Hib as a causative organism. The ICD-10-AM/ICD-10 code used to identify presumed Hib cases was G00.0 (Haemophilus meningitis). The ICD-10-AM/ICD-10 codes for H. influenzae pneumonia, H. influenzae septicaemia, H. influenzae infection and acute epiglottitis were not included as these were thought to be insufficiently specific for invasive H. influenzae type b disease.

Secular trends

During the three years from January 2003 to December 2005, a total of 51 invasive Hib infections were notified. The average annual notification rate was 0.08 per 100,000 population (Table 3). A median of one case (range 0–5) was notified per month (Figure 1). There were 57 hospitalisations (average annual rate 0.10 per 100,000) recorded as Haemophilus meningitis, with a median of two cases (range 0–4) hospitalised per month.

Figure 1. Haemophilus influenzae type b (Hib) notifications and Haemophilus meningitis hospitalisations for all ages, Australia, 1993 to 2005,* by month of diagnosis or admission

Figure 1. Haemophilus influenzae type b (Hib) notifications  and <em>Haemophilus</em> meningitis  hospitalisations for all ages, Australia,  1993 to 2005, by month of diagnosis or admission

* Notifications where the month of diagnosis was between January 1993 and December 2005; hospitalisations where the month of admission was between 1 July 1993 and 30 June 2005.

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Severe morbidity and mortality

Overall ,within each age group, there were similar numbers of hospitalisations with H. influenzae meningitis and notifications of invasive Hib disease recorded (Table 3). Over the review period, a total of 780 hospital bed days (average 260 days per year) was recorded for patients with Haemophilus meningitis. The median length of stay for hospitalisations with a principal diagnosis of Haemophilus meningitis was 11 days.

In the two years 2003 to 2004, H. influenzae meningitis was certified as the underlying cause of death in one person (who was aged over 85 years) (Table 3). There were two deaths in notified cases reported to NNDSS between 2003 and 2005 (both in 2003). One case was in an infant and the other in a man aged 60–69 years.

Table 3. Haemophilus influenzae type b (Hib) notifications, Hib meningitis hospitalisations and deaths, Australia, 2002 to 2005,* by age group

Age group
(years)
Notifications
3 years
(2003–2005)
Hospitalisations
3 years
(July 2002–June 2005)
LOS per admission
(days)
Deaths
2 years
(2003–2004)
  n Rate n n(§) Rate Rate(§) Median(§) n Rate
0–4
23
0.61
35
26
0.92
0.68
10.0 (10.0)
0
0.00
5–14
8
0.10
4
4
0.05
0.05
8.5 (8.5)
0
0.00
15–24
1
0.01
3
1
0.04
0.01
n.p.
0
0.00
25–59
9
0.03
4
3
0.01
0.01
16.5 (19.0)
0
0.00
60+
10
0.09
11
9
0.11
0.09
15.0 (15.0)
1
0.01
All ages
51
0.08
57
43
0.10
0.07
11.0 (11.0)
1
0.003

* Notifications where the month of diagnosis was between January 2003 and December 2005; 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).

n.p. Not published due to small cell sizes.

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Age and sex

Hospitalisations for presumed Hib disease and notifications were higher in males than in females, with a male:female ratio of 1.21:1 and 1.33:1, respectively. In children aged 0–4 years, there were 23 H. influenzae meningitis cases. Overall, children aged 0–4 years accounted for 45% (23/51) of all notifications, 61% (35/57) of all presumed meningitis hospitalisations but no deaths (Table 3). The age-specific notification rate closely matched the age-specific H. influenzae meningitis hospitalisation rate (Figure 2).

Figure 2. Haemophilus influenzae type b notification and Haemophilus meningitis hospitalisation rates, Australia, 2003 to 2005,* by age at admission

Figure 2. Haemophilus influenzae type b  notification and <em>Haemophilus</em> meningitis  hospitalisation rates, Australia, 2003 to 2005, by age at admission

* Notifications where the month of diagnosis was between January 2003 and December 2005;
hospitalisations where the month of admission was between 1 July 2003 and 30 June 2005.

Since 1993, all measures of invasive Hib disease in children aged 0–4 years, who had the highest disease incidence prior to the introduction of vaccination (and are the most highly immunised), have progressively fallen, though less steeply in recent years (Figure 3). In this age group, the average number of annual notified cases has decreased from approximately 27 in the late 1990s to approximately 10 between 2000 and 2002 to approximately 7 between 2003 and 2005. Six deaths were recorded in this age group in 1993 but none since 1999.

Figure 3. Haemophilus influenzae type b (Hib) notification and ‘presumed invasive Haemophilus influenzae type b’ hospitalisation rates and numbers of deaths* for children aged 0–4 years, Australia, 1993 to 2005

Figure 3. Haemophilus influenzae type b notification and presumed invasive Haemophilus  influenzae type b hospitalisation rates and numbers of deaths for  children aged 0-4 years, Australia,  1993 to 2005

* Hospitalisation and deaths include those for Haemophilus meningitis for the period up to 30 June 2005 (hospitalisations) and 31 December 2004 (deaths).

† Notifications where the month of diagnosis was between January 1993 and December 2005;
hospitalisations where the month of separation was between 1 July 1993 and 30 June 2005; deaths where the death was recorded between January 1993 and December 2004.

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Vaccination status

Completion of the vaccination status field was expected for all notifications of Hib born after 31/12/1987 in NNDSS during 2003–2005. Overall, 94% of 31 cases had this field completed for this period. Of 20 cases classified as fully (n=10) or partially vaccinated (n=5), vaccination was validated from the Australian Childhood Immunisation Register or written records in 12/20 (60%). Among 14 cases 1–7 years of age notified between 2003 and 2005, nine (64%) were not vaccinated.

Geographical variation

As in previous years, there was little variation in notification and hospitalisation rates between the states and territories, except for the Northern Territory, where notification rates were substantially higher than other jurisdictions, but the absolute number of cases was small (Appendices 2 and 3).

Comment

This report is the first in the series of NCIRS vaccine preventable diseases reports to exclude hospitalisations recorded as epiglottitis from presentation as a measure of Hib disease. This is because a review of hospitalisations coded as epiglottitis in Sydney from 1998 to 2000 showed none of these hospitalisations had Hib isolated from a sterile site, with one due to Streptococcus pneumoniae and a substantial proportion (32%) of incorrect coding.30 Hospitalisation data now includes meningitis only, although as serotype-specific hospitalisation data are still not available, these cases, even when Haemophilus meningitis is the primary diagnosis, could be due to other serotypes.

In 2000, a new Hib immunisation schedule began in Australia with all children receiving PRP-OMP vaccine at 2 and 4 months of age, with a booster at 12 months of age. In this review period, vaccination status data are available for the first time and indicate that, consistent with the very high immunisation coverage reported for Hib vaccine, approaching 95%, the majority of confirmed Hib cases are occurring in unimmunised children. This is consistent with high vaccine effectiveness but also indicates that unimmunised children remain at risk of severe disease despite population herd immunity. Further, there is no evidence of any increase in Hib cases in older age groups, although the first cohort of children eligible to receive Hib vaccine are now approaching 20 years of age. Incidence of Hib may not decrease much more than the very low incidence now reached, as it is consistent with the lowest disease rates reported internationally,62,63 and in contrast to the increase in Hib cases seen in the United Kingdom where three doses of Hib vaccine were scheduled at 2, 3 and 4 months of age with no later booster.64 The rarity of invasive Hib disease emphasises the importance of laboratory confirmation of all suspected cases, ideally by typing with polymerase chain reaction (PCR) in a reference laboratory.

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