Vaccine Preventable Diseases in Australia, 2005 to 2007

3.2 Haemophilus influenzae type b disease

Page last updated: 24 December 2010

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. Based on their capsular polysaccharide, H. influenzae can be further characterised into six types designated a to f; there are also non-typeable strains. Hib has most often been associated with invasive disease, and before Hib vaccines became available, caused at least 95% of invasive disease due to H. influenzae in children.1–3 Prior to the introduction of vaccination, the most common manifestation of invasive Hib disease globally was meningitis, with children aged <18 months most at risk.1,2 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.4 Survivors of Hib meningitis commonly had neurological sequelae such as deafness and intellectual impairment.1–3 Epiglottitis was the other major category of infection with particularly high rates observed among non-Indigenous Australians, most often occurring in children aged >18 months. Other manifestations of Hib disease include cellulitis, septic arthritis, pneumonia, pericarditis, osteomyelitis and septicaemia.

Case definitions

Notifications

See Appendix 6.6 for pre-2004 definition

Invasive Hib infection – national definition from January 2004:5

Only confirmed cases are notifiable.

  1. Isolation of Haemophilus influenzae type b from a normally sterile site where typing has been confirmed at an approved reference laboratory; or
  2. 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 considered insufficiently specific for invasive H. influenzae type b disease.

Top of page

Secular trends

During the 2 years from January 2006 to December 2007, a total of 39 invasive Hib infections were notified. The average annual notification rate was 0.09 per 100,000 population (Table 3.2.1). A median of 1 case (range 0–6) was notified per month (Figure 3.2.1). There were 25 hospitalisations (average annual rate 0.06 per 100,000) recorded as Haemophilus meningitis, with a median of 0.5 cases (range 0–5) hospitalised per month.

Table 3.2.1: Haemophilus influenzae type b notifications, Haemophilus meningitis hospitalisations and deaths, Australia, 2005 to 2007,* by age group

Age group
(years)
Haemophilus influenzae type b notifications
2 years
(2006–2007)
Haemophilus meningitis hospitalisations
2 years
(July 2005–June 2007)
LOS per
admission
(days)
Haemophilus meningitis deaths
2 years
(2005–2006)
n
Rate
n
(§)
Rate
(§)
Median
(§)
n
Rate
0–4
21
0.80
15
(14)
0.58
(0.54)
11.0
(10.0)
0
5–14
4
0.07
0
(0)
(–)
(–)
0
15–24
2
0.03
0
(0)
(–)
(–)
0
25–59
5
0.02
5
(4)
0.02
(0.02)
10.0
(9.5)
0
60+
7
0.09
5
(4)
0.07
(0.06)
13.0
(16.5)
1
0.01
All ages
39
0.09
25
(22)
0.06
(0.05)
11.0
(10.5)
1
<0.005

* Notifications where the date of diagnosis was between January 2006 and December 2007; hospitalisations where the date of separation was between July 2005 and June 2007; deaths where the death was recorded between January 2005 and December 2006.

† LOS = length of stay in hospital.

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

§ Principal diagnosis (hospitalisations).

Top of page

Figure 3.2.1: Haemophilus influenzae type b notifications and Haemophilus meningitis hospitalisations for all ages, Australia, 1993 to 2007,* by month of diagnosis or admission

Figure 3.2.1: Haemophilus influenzae type b notifications and Haemophilus meningitis hospitalisations for all ages, Australia, 1993 to 2007, by month of diagnosis or admission

* Notifications where the date of diagnosis was between January 1993 and December 2007; hospitalisations where the date of admission was between July 1993 and June 2007.

Top of page

Figure 3.2.2: Haemophilus influenzae type b notification and Haemophilus meningitis hospitalisation rates and numbers of deaths* for children aged 0–4 years, Australia, 1993 to 2007

Figure 3.2.2: Haemophilus influenzae type b notification and Haemophilus meningitis hospitalisation rates and numbers of deaths for children aged 0-4 years, Australia, 1993 to 2007

* Hospitalisations and deaths coded as Haemophilus meningitis for the period up to June 2007 (hospitalisations) and December 2006 (deaths).

† Notifications where the date of diagnosis was between January 1993 and December 2007; hospitalisations where the date of separation was between July 1993 and June 2007; deaths where the death was recorded between January 1993 and December 2006.

Top of page

Severe morbidity and mortality

Hib morbidity and mortality data are drawn from three data sources: Haemophilus meningitis hospitalisation data, the AIHW National Mortality Database and NNDSS (Table 3.2.1).

Notification and hospitalisation rates by age group were broadly concordant, with the exception of the 5–24 years age group in whom no hospitalisations due to Haemophilus meningitis were reported between July 2005 and June 2007 despite notifications over this period. The total number of hospital bed days recorded for patients with this diagnostic code was 322 (average 161 bed days per year). The overall median length of stay for hospitalisations with a principal diagnosis of Haemophilus meningitis was 10.5 days.

Mortality data can be drawn from these three databases. However, these datasets are not linked and data are available from differing time periods – the AIHW National Mortality Database data from 2005–2006, the hospitalisation data from 2005/2006–2006/2007, and NNDSS from 2006–2007. Two deaths attributable to Hib were reported from these data sources during these overlapping reporting periods. In 2005, the death of a person aged ≥85 years due to Haemophilus meningitis was recorded in the AIHW National Mortality Database. This record appeared to correspond to the only case of Haemophilus meningitis hospitalisation that resulted in death in 2005/2006. Among the invasive Hib disease cases notified to NNDSS in 2006 and 2007, there was 1 death in an Indigenous female infant in 2006.

Top of page

Age and sex

Discrepancies in case ascertainment between data sources were apparent but also expected given that only notification data are specific for serotype b. The rate of hospitalisations coded as Haemophilus meningitis was higher in males than in females, with a male:female ratio of 3.2:1. However, the rate of notifications to the NNDSS for invasive Hib disease was higher in females than in males with a male:female ratio of 0.63:1. These ratios were dominated by those from the 0–4 years age group which represented most of the notifications.

In children aged 0–4 years, there were 21 notifications of invasive Hib disease, accounting for 53.8% (21/39) of all notifications. Sixty-seven per cent (15/25) of all Haemophilus meningitis hospitalisations were in this age group with no reported deaths (Table 3.2.1). Ten of the 21 notifications and 9 of the 15 hospitalisations occurred in infants aged <1 year. Three of the 21 notifications and 4 of the 15 hospitalisations occurred in infants aged 1 to <2 years. For most age groups, the age-specific notification rates approximated the age-specific Haemophilus meningitis hospitalisation rates.

Since 1993 and after the introduction of universal childhood Hib immunisation, all measures of invasive Hib disease in children aged 0–4 years, who previously had the highest disease incidence, have progressively fallen, though less steeply in recent years (Figure 3.2.2). In this age group, the number of cases notified annually has decreased from approximately 27 in the late 1990s to remain at between 5 and 13 cases annually from 2000 to 2007. It should be noted that there are a number of caveats around the available death data. Between 1993 and 2007, NNDSS recorded 5 deaths as due to invasive Hib disease in the 0–4 years age group, whereas 12 deaths were coded as due to Haemophilus meningitis in the AIHW National Mortality Database for this age group over the same time period.

Top of page

Vaccination status

Completion of the vaccination status field in NNDSS was expected for all notifications of invasive Hib in subjects born after 31 December 1987. Twenty-six of 27 cases (96.3%) who were born after this date had this field completed; however, 2 of these 26 were entered as ‘unknown’. Among the 24 cases whose vaccination status was known, 19 had this validated from the Australian Childhood Immunisation Register or written records, accounting for all cases classified as fully (n=12) or partially vaccinated (n=2), and 5 of the 10 cases classified as non-vaccinated.

Of the 14 cases notified in children aged 1–<10 years in 2006 and 2007, 2 (14%) were in partially vaccinated children aged 1 year, 3 (21%) were in non-vaccinated children aged 1–2 years, and vaccination status was unknown for 1 case (7%) aged 5 years. The other 8 notifications were in children who were reported as fully vaccinated; their median age was 3 years.

There were 10 notifications in infants aged <1 year, of whom 6 were in infants aged <6 months. Of these 6 notifications, 1 case was reported as fully vaccinated for age, 4 as unvaccinated, and vaccination status was unknown for 1 case.

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 remained small (Appendices 6.2 and 6.3).

Top of page

Comment

This report is the second in the series of NCIRS national reports on vaccine preventable diseases that excludes hospitalisations recorded as epiglottitis as a measure of Hib disease. This is because a previous 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%) a result of incorrect coding.6 Hospitalisation data now includes Haemophilus meningitis only, and, although type-specific hospitalisation data are still not available, these cases, even when Haemophilus meningitis is the primary diagnosis, could in fact be due to non-type b disease or non-typeable H. influenzae. (See Methods section for further details regarding the limitations of the data sources utilised.)

There have been three eras of Hib vaccination in Australia.3 During the first era from 1993 until June 2000, all children resident in the Northern Territory received 2 doses of a vaccine containing the Hib component conjugated to an outer membrane protein derived from Neisseria meningitidis (PRP-OMP) at 2 and 4 months of age, with a booster at 12 months of age, whereas all children in the remaining states and territories received 3 doses of a vaccine containing the Hib component conjugated to a mutant diphtheria toxin (HbOC) at 2, 4 and 6 months of age, with a booster at 18 months of age. In 2000, the second era began in Australia with all children receiving 2 doses of PRP-OMP vaccine at 2 and 4 months of age, with a booster at 12 months of age. The third era commenced during this review period, in November 2005. All children resident in the Northern Territory, Queensland, Victoria and South Australia and Indigenous children in Western Australia were administered PRP-OMP vaccine with 2 doses at 2 and 4 months of age and a booster dose at 12 months of age, while all children in New South Wales, the Australian Capital Territory and Tasmania and non-Indigenous children in Western Australia received a vaccine containing the Hib component conjugated to a tetanus toxoid protein (PRP-T) with 3 doses administered at 2, 4 and 6 months of age and a booster dose at 12 months of age.3

Vaccination status data indicate that, consistent with the very high immunisation coverage reported for Hib vaccine (approaching 95%), many of the 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 25 years of age. An assessment of preventable cases of invasive Hib disease in vaccine eligible children between July 2000 and December 2005 reported that the proportion of cases identified as preventable did not differ significantly between Indigenous and non-Indigenous children.3 Furthermore, this report indicated that approximately 60% of invasive Hib disease cases were preventable as they occurred among those who were either not immunised or not fully immunised.3

Incidence of invasive Hib disease in Australia may not decrease much more than the very low incidence now reached, as it is consistent with the lowest disease rates reported internationally.7,8 The UK experienced a resurgence of Hib infections 8 years following introduction of an accelerated primary schedule, with disease control re-established following a national catch-up immunisation campaign and the addition of a routine 4th (booster) dose in the 2nd year of life.9,10 This experience highlights the importance of continued post-licensure surveillance of vaccine preventable diseases. The rarity of invasive Hib disease re-emphasises the importance of laboratory confirmation of all suspected cases, ideally by typing with polymerase chain reaction (PCR) in a reference laboratory.

Top of page

References

1. McIntyre PB, Leeder SR, Irwig LM. Invasive Haemophilus influenzae type b disease in Sydney children 1985–1987: a population-based study. Med J Aust 1991;154(12):832–837.

2. Gilbert GL, Clements DA, Broughton SJ. Haemophilus influenzae type b infections in Victoria, Australia, 1985 to 1987. Pediatr Infect Dis J 1990;9(4):252–257.

3. Wang H, Deeks S, Glasswell A, McIntyre P. Trends in invasive Haemophilus influenzae type b disease in Australia, 1995–2005. Commun Dis Intell 2008;32(3):316–325.

4. Hanna J. The epidemiology and prevention of Haemophilus influenzae infections in Australian aboriginal children. J Paediatr Child Health 1992;28(5):354–361.

5. Communicable Diseases Network Australia. Surveillance case definitions for the Australian National Notifiable Diseases Surveillance System. 2004. Available from: http://www.health.gov.au/internet/main/publishing.nsf/Content/cdna-casedefinitions.htm Accessed on 17 February 2009.

6. Wood N, Menzies R, McIntyre P. Epiglottitis in Sydney before and after the introduction of vaccination against Haemophilus influenzae type b disease. Intern Med J 2005;35(9):530–535.

7. Peltola H. Worldwide Haemophilus influenzae type b disease at the beginning of the 21st century: global analysis of the disease burden 25 years after the use of the polysaccharide vaccine and a decade after the advent of conjugates. Clin Microbiol Rev 2000;13(2):302–317.

8. Bisgard KM, Kao A, Leake J, Strebel PM, Perkins BA, Wharton M. Haemophilus influenzae invasive disease in the United States, 1994–1995: near disappearance of a vaccine-preventable childhood disease. Emerg Infect Dis 1998;4(2):229–237.

9. McVernon J, Andrews N, Slack MP, Ramsay ME. Risk of vaccine failure after Haemophilus influenzae type b (Hib) combination vaccines with acellular pertussis. Lancet 2003;361(9368):1521–1523.

10. UK Department of Health. Haemophilus influenzae type b (Hib). In: Salisbury D, Ramsay M, Noakes K, eds. Immunisation against infectious disease – ‘The Green Book’. London: TSO (The Stationery Office), 2006. Available from: http://www.dh.gov.uk/en/Publichealth/Healthprotection/Immunisation/Greenbook/DH_4097254 Accessed on 5 March 2009.

Document download

This publication is available as a downloadable document.

Vaccine Preventable Diseases in Australia, 2005 to 2007(PDF 1217 KB)