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

Pertussis

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

Pertussis (whooping cough) is an acute illness, caused by the Bordetella pertussis bacterium, involving the respiratory tract. The illness begins with an irritating cough that gradually becomes paroxysmal and lasts for one to two months or longer. Paroxysms are characterised by repeated violent coughs and are followed by a characteristic crowing or high-pitched inspiratory whoop. Infants less than six months of age, adolescents and adults often have fewer classical symptoms without paroxysms or whoop.44

Case definitions

See Appendix 6 for pre 2004 definition

National definition from January 2004:11

Confirmed cases are those with definitive laboratory evidence, laboratory suggestive evidence and clinical evidence, or clinical evidence and an established epidemiological link to a confirmed case. Probable cases require clinical evidence (as below) only and are also notified.

a) Laboratory definitive evidence

  • Isolation of B. pertussis from a clinical specimen or detection of B. pertussis by nucleic acid testing

b) Laboratory suggestive evidence

  • Seroconversion or significant increase in antibody levels or fourfold or greater rise in titre to B. pertussis, in absence of recent vaccination; or
  • Single high IgA titre to whole cells; or
  • Detection of B. pertussis antigen by immunofluorescence assay (IFA).

c) Clinical evidence

  • A coughing illness lasting two or more weeks; or
  • Paroxysms of coughing or inspiratory whoop or post-tussive vomiting.

Hospitalisations and deaths

The ICD-10-AM/ICD-10 code A37 (whooping cough) was used to identify hospitalisations and deaths.

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Secular trends

There were 25,035 notifications of pertussis received by the National Notifiable Diseases Surveillance System (NNDSS) with dates of onset in the three years January 2003 to December 2005 (average annual rate 41.5 per 100,000) (Table 13). A median of 635 cases was notified each month (range 217–1,474). Both 2004 and 2005 were epidemic years, with 8,750 and 11,191 notifications, respectively, compared with 5,094 notifications in 2003. Epidemic peaks have occurred every three to four years since national notifications became available in 1991. The national notification rate was 55.1 per 100,000 in 2005 and was the second highest national rate recorded since 1993, after the 1997 national rate of 58.9 per 100,000 with 10,828 notified cases. A clear seasonal pattern remained apparent, with the highest number of notifications in the spring and summer months (between August and February) each year between 1993 and 2005 (Figure 25).

Hospitalisations followed a similar pattern to notifications. There were 1,319 hospital separations coded as pertussis during the review period, 359 in 2002/2003, 373 in 2003/2004 and 587 in 2004/2005 (Table 13 and Appendix 3). The median number of pertussis hospitalisations per month was 37 (range 17–65). The average annual national hospitalisation rate was 2.2 per 100,000 for this reporting period, compared with 3.3 per 100,000 for the previous two years 2000/2001 to 2001/2002.3

Figure 25. Pertussis notifications and hospitalisations, Australia, 1993 to 2005,* by month of diagnosis or admission

Figure 25. Pertussis  notifications and hospitalisations, 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 between1 July 1993 and 30 June 2005.

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Severe morbidity and mortality

There were 8,038 hospital bed days recorded with an ICD-10-AM code for pertussis between July 2002 and June 2005 (2,500 for 2002/2003, 2,163 for 2003/2004 and 3,375 for 2004/2005). The median length of stay per admission was four days (Table 13). Of the 1,319 hospitalisations, 1,034 (78%) had a principal diagnosis of pertussis (average annual rate 1.7 per 100,000). The discharge diagnosis code A37.0 (B. pertussis) was recorded for 596 (45%) hospitalisations and was the principal diagnosis for 470 (79%) of these. Bordetella parapertussis (A37.1) was recorded for 14 hospitalisations, and other Bordetella species (A37.8) for 22 hospitalisations. The remaining 687 (52%) hospitalisations were coded as whooping cough (organism unspecified – A37.9), and this was the principal diagnosis for 544 (79%) of these.

For the two years 2003 to 2004, two deaths were recorded where pertussis was the underlying cause (Table 13). Both occurred in 2004; one case was one month of age and the other was a 95 year old. Between 1993 and 2002, there were 16 deaths attributed to pertussis: all but one were younger than 12 months of age; six occurred in 1997.1–3

There were three deaths in notified cases reported to NNDSS between 2003 and 2005. Two of these are the cases identified in the 2004 death data and the other was a one month old case notified in 2005.

Table 13. Pertussis notifications, 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 (§) Rate (§) Median(§) n Rate
0–4
1,688
44.5
764
692
20.1
18.2
3.0 (3.0)
1
0.04
5–14
3,551
43.6
104
78
1.3
1.0
2.0 (2.0)
0
0.00
15–24
3,263
39.3
37
28
0.5
0.3
2.0 (2.0)
0
0.00
25–59
12,738
43.2
241
150
0.8
0.5
4.0 (3.0)
0
0.00
60+
3,789
36.0
173
86
1.7
0.8
7.0 (6.0)
1
0.01
All ages||
25,035
41.5
1,319
1,034
2.2
1.7
4.0 (3.0)
2
0.01

* 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.

|| Includes cases with unknown ages.

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Age and sex distribution

The highest notification rates were seen in infants aged less than one year (Figure 26), with annual average rates of 113.1 per 100,000 in 2004 and 100.8 per 100,000 in 2005. In the three year review period, infants aged less than one year accounted for 3% of all notifications (n=763) but 50% of hospitalisations (n=655). The average hospitalisation rate for infants was 88.1 per 100,000 in this reporting period compared with 154.1 per 100,000 for the previous two years, 2000/2001–2001/2002 (Figure 27 and Figure 28).3

The 10–19 year age group accounted for 19% of pertussis notifications in 2003–2005 (n=4,824) and 8% of all hospitalisations (n=100). The 10–19 year age group had very high notification rates in 2003 and 2004, similar to recent patterns of notifications in this age group3 and second only to the notification rate in infants aged less than 1 year. In 2005, there was a decline in the notification rate for 10–19 year olds to 41.4 per 100,000, with the notification rate in this age group falling below that of people aged 20–59 years and 60 years and over.

People aged 20–59 years (adults) accounted for 56% of notifications (n =13,902) and 19% of hospitalisations (n =252, with an average annual hospitalisation rate of 0.8 per 100,000, the same as in the previous reporting period).3 Elderly people aged 60 years and over accounted for 15% of notifications (n =3,789) and 13% of hospitalisations (n=173). The notification rates in those aged 20–59 years and 60 years and over have both recently risen to a record high of 62 notifications per 100,000 population, which is in contrast to the relatively steady annual rates previously seen in these age groups. The proportion of notifications in these groups has also been increasing, from 35%–45% between 1993 and 1998 to 83% in 2005.

The overall male:female ratio was 1:1.3 for notifications and 1:1.1 for hospitalisations. Higher rates among females were apparent in most age groups for notifications and hospitalisations. The exception to this was for hospitalisations in people aged 70 years and over, where the male:female ratio was 1.6:1.

Figure 26. Pertussis notification rates, Australia, 1993 to 2005,* by age group

Figure 26. Pertussis  notification rates, Australia,  1993 to 2005, by age group

* Notifications where date of diagnosis was between 1 January 1993 and 31 December 2005.

Figure 27. Pertussis hospitalisation rates, Australia, 1993 to 2005,* by age group

Figure 27. Pertussis  hospitalisation rates, Australia,  1993 to 2005, by age group

* Hospitalisations where date of separation was between 1 July 1993 and 30 June 2005.

Figure 28. Pertussis hospitalisation rates, Australia, 1993 to 2005,* by age group (excluding <1 yrs)

Figure 28. Pertussis  hospitalisation rates, Australia,  1993 to 2005, by age grou

* Hospitalisations where date of separation was between 1 July 1993 and 30 June 2005.

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Geographical distribution

Periodic epidemics of pertussis occur in Australia at intervals of three to four years. However, the frequency and length of the epidemic cycles varies within states, particularly in the less populated and/or geographically isolated states and territories, such as Western Australia, the Northern Territory and Tasmania. During the study period, there was a large variation in notification (Appendix 2) and hospitalisation rates (Appendix 3) between regions and years. The Australian Capital Territory experienced a pertussis epidemic in 2003 and was the only state or territory to do so, with a notification rate of 110.4 per 100,000 population. In 2004, an elevated notification rate of 106.2 per 100,000 population signified a pertussis epidemic in Western Australia, seven years after the previous epidemic in this state. South Australia (97.7 per 100,000), the Australian Capital Territory (96.9 per 100,000), New South Wales (85.5 per 100,000), the Northern Territory (45.4 per 100,000) and Queensland (44.8 per 100,000) all experienced elevated notification rates in 2005.

Vaccination status

Completion of the vaccination status field was expected for all pertussis notifications aged under 15 years in NNDSS during 2003–2005. Overall, 72% of cases aged 0–14 years of age had this field completed for this period. Field completion rates varied by age group, ranging from 58% completion for cases aged 10–14 years, to 96% for those aged 6–11 months. The percentage of cases fully and partially vaccinated for age was calculated for people with a known vaccination status aged less than 9 years. The number of cases fully vaccinated for age rose from 37% in infants less than 6 months of age to 70% in those aged 1–4 years (Table 14).

Table 14. Vaccination status of notified pertussis cases aged six months to nine years, Australia, 2003 to 2005,* by age group

Age group
Vaccination status
(2003–2005)
Fully vaccinated for age Partially vaccinated for age
n % n %
0–5 months
188
37
61
12
6–11 months
116
60
30
15
1–4 years
454
70
14
2
4–9 years
499
61
51
6

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

† Percentage of those with a known vaccination status.

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Comment

Since 1993, pertussis has caused the greatest morbidity of any disease preventable by vaccines recommended for children on the National Immunisation Program (NIP) schedule. The highest numbers of pertussis notifications were seen in 2005, with many jurisdictions experiencing an epidemic in that year, followed by 1997 and 2001. Traditionally, hospitalisations in infants aged less than one year have exceeded notifications, indicating that notification rates tend to underestimate pertussis incidence.3,149 However, in the 2003–2005 period, there were more notifications than hospitalisations in this age group, which may be a reflection of the increased use of PCR to diagnose pertussis in children. In children, hospitalisations coded as whooping cough have been shown to have a high correlation with clinical pertussis.19 The high proportion (greater than 50%) of hospitalised cases aged less than one year is consistently observed each year and demonstrates the increased morbidity of pertussis in this age group.

Nationally, the highest notification rates up to 1998 inclusive were among children aged less than one year, followed closely by children aged 5–9 and 10–19 years (Figure 26). Since 1999, notification rates have fallen significantly among 5–9 year olds, reflecting the impact of the fifth dose of pertussis vaccine, introduced since 1994 for four year olds because of waning immunity over time.150 The number of cases reported to be vaccinated for age may be an over-estimate, as the calculation did not include those with an “unknown” status (it is more likely that those recorded as unknown were not vaccinated). Studies show that a primary three-dose course of acellular pertussis vaccine provides 80%–85% protection.76

High incidence rates among 10–19 year olds continued to occur in 2003 and 2004. The susceptibility of this cohort is explained by a combination of low historical coverage (whole-cell vaccine safety concerns in the 1980s) and waning immunity (cohort not eligible for school entry booster dose).150,151 There has been a downward trend in the notification rate for this age group since 2002 and a sharper decline in the rate since 2004. This is likely to reflect the impact of the fifth dose of pertussis vaccine reaching this older cohort and the impact of an adolescent booster vaccine (dTpa), introduced in November 2003. In response to the high incidence rate in adolescents, both New South Wales and Western Australia conducted whole of high school dTpa vaccination programs in 2004. The combined incidence rate for 10–19 year olds in these states decreased from an average of 85.7 per 100,000 population for 1999–2003 to 37.2 per 100,000 population in 2005.152 As Australian school-based dTpa programs mature and successive cohorts are vaccinated in future years, pertussis in adolescents should become well controlled, as occurred in 5–9 year olds following the introduction of the preschool booster.

In essence, pertussis is now a problem in two broad age groups: infants with the highest notification and hospitalisation rates, particularly those under 6 months of age who are too young to have received two or more doses of DTPa, and people aged 20 years and over, who account for 80% of pertussis notifications. The latter could be partly related to the increased use, especially in adults, of serology as a diagnostic tool (NNDSS data from 2000–2005 shows an increasing percentage of notifications diagnosed by serology (Quinn H et al, NCIRS, unpublished data)). In addition, recommendations for use of pertussis vaccine in adults make it increasingly likely that clinicians will consider pertussis as a potential cause of chronic cough in adults. Hospitalisations in adults are most likely to be related to complications, but could also be falsely inflated because of coding errors. Although severe morbidity and mortality are less likely in adults, increased circulation of pertussis can facilitate transmission to susceptible infants who are too young to be vaccinated.153–155 The recent increase in the incidence and burden of pertussis notifications in persons aged 60 years and over warrants further investigation. As with parents, grandparents are an important source of pertussis transmission to infants.153 It is also unclear whether the morbidity of pertussis in older people is more severe, or if complications are more likely to occur. With this in mind, it is interesting to note that two of eight pertussis deaths in the past five years have been recorded in people aged 60 years and over.3 The current adult pertussis immunisation strategy in Australia is aimed at cocooning infants, by recommending immunisation in adults who are most likely to come into contact with them (such as family members, health care workers and child care workers). It is hoped that, in time, this may have an impact on neonatal cases.

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