Vaccine Preventable Diseases in Australia, 2005 to 2007

3.9 Pertussis

Page last updated: 24 December 2010

Pertussis (whooping cough), caused by the Bordetella pertussis bacterium, is an acute illness, involving the respiratory tract. The typical illness begins with an irritating cough that becomes paroxysmal, lasts for 1–2 months or longer and may be associated with post-tussive vomiting. Paroxysms are characterised by repeated violent coughs, followed by a characteristic crowing or high-pitched inspiratory whoop. Infants less than 6 months old, adolescents and adults often have fewer classical symptoms without paroxysms or whoop.1

Case definitions

Notifications

See Appendix 6.6 for pre-2004 definition

National definition from January 2004:2

Both confirmed and probable cases are notifiable.

Confirmed cases are those with definitive laboratory evidence; or laboratory suggestive evidence together with clinical evidence; or clinical evidence together with an established epidemiological link to a confirmed case with laboratory evidence.

  1. Laboratory definitive evidence
    • Isolation of Bordetella pertussis from a clinical specimen or detection of B. pertussis by nucleic acid testing.
  2. Laboratory suggestive evidence
    • Seroconversion or significant increase in antibody level or 4-fold 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).
  3. Clinical evidence
    • A coughing illness lasting 2 or more weeks; or
    • Paroxysms of coughing or inspiratory whoop or post-tussive vomiting.
    • Probable cases require clinical evidence only. The clinical evidence required is
    • A coughing illness lasting 2 or more weeks; AND
    • 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 16,321 notifications of pertussis received by the NNDSS with dates of onset in the 2 years between January 2006 and December 2007 (Table 3.9.1). Of these 16,321 notifications, 13,880 (85%) were confirmed cases, and 2,441 (15%) were probable cases. A median of 547 cases were notified each month (range 275–1,596). Epidemic peaks have occurred every 3–4 years since national notifications became available in 1991. 2006 was an epidemic year with 10,992 notifications, dropping to 5,329 notifications in 2007. In 2006, the national notification rate was 53.1 per 100,000, the third highest national rate recorded since 1993, after 1997 (58.1 per 100,000) and 2005 (54.9 per 100,000). A clear seasonal pattern was apparent, with the highest number of notifications in the late winter and summer months (between August and February) each year between 1995 and 2007 (Figure 3.9.1).

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Figure 3.9.1: Pertussis notifications and hospitalisations, Australia, 1995 to 2007,* by month of diagnosis or admission

Figure 3.9.1:  Pertussis notifications and hospitalisations, Australia, 1995 to 2007, by month of diagnosis or admission

Note varying scales between notifications and hospitalisations.

* Notifications where the date of diagnosis was between January 1995 and December 2007; hospitalisations where the date of admission was between January 1995 and June 2007.

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Hospitalisations followed a similar pattern to notifications. The median number of pertussis hospitalisations per month was 32 (range 13–91) and the average annual national hospitalisation rate was 2.1 per 100,000 for this reporting period, similar to the previous 3 years 2002/2003 to 2004/2005 (2.2 per 100,000).

Table 3.9.1: Pertussis notifications, hospitalisations and deaths, Australia, 2005 to 2007,* by age group

Age group
(years)
Notifications
2 years
(2006–2007)
Hospitalisations
2 years
(July 2005–June 2007)
LOS per admission
(days)
Deaths
2 years
(2005–2006)
n Rate n (§) Rate (§) Median (§) n Rate
0–4
596
22.6
340
(284)
13.1
(10.9)
2.0
(2.0)
1
0.04
5–14
765
13.9
31
(19)
0.6
(0.3)
2.0
(2.0)
0
15–24
1,503
25.8
20
(10)
0.4
(0.2)
4.0
(2.0)
0
25–59
9,145
45.1
227
(130)
1.1
(0.6)
4.0
(4.0)
0
60+
4,310
57.5
264
(151)
3.6
(2.1)
7.0
(6.0)
2
0.03
All ages
16,321||
39.1
882
(594)
2.1
(1.4)
4.0
(3.0)
3
0.01

* 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).

|| Includes 2 cases with unknown age.

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Severe morbidity and mortality

From July 2005 to June 2007, there were 882 episodes of hospitalisation with a pertussis diagnosis (ICD-10-AM code A37) (457 in 2005/2006 and 425 in 2006/2007). The median length of stay per hospital admission was 4 days (Table 3.9.1) and the total number of bed days for the period was 6,214. Of the 882 hospitalisations, 594 (67%) had a principal diagnosis of pertussis (average annual rate 1.4 per 100,000). The discharge diagnosis code A37.0 (B. pertussis) was recorded for 439 (50%) hospitalisations and was the principal diagnosis for 298 (68%) of these. Bordetella parapertussis (A37.1) was recorded for 9 hospitalisations, and other Bordetella species (A37.8) for 16. The remaining 417 (47%) hospitalisations were coded as whooping cough (organism unspecified – A37.9), which was the principal diagnosis in 285 (68%) cases.

For the 2 years 2005–2006, 3 deaths were recorded where pertussis was the underlying cause (Table 3.9.1). While all the 3 deaths were recorded in 2006, 2 of these 3 recorded deaths in fact occurred in late December 2005, with one 1 month of age and two 93 and 94 years of age. Between 1993 and 2004, there were 18 deaths with pertussis recorded as the underlying cause of death. All but two were younger than 12 months of age; six occurred in 1997.3–6

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Age and sex distribution

The highest notification rates were seen in infants aged <1 year (Figure 3.9.2), with annual average rates of 59.1 per 100,000 in 2006 and 46.6 per 100,000 in 2007. In the 2-year review period, infants aged <1 year accounted for 1.8% of all notifications (n=286) but 34% of hospitalisations (n=300). The average hospitalisation rate for infants was 57.0 per 100,000 in this reporting period compared with 88.1 per 100,000 for the previous 3 years, 2002/2003–2004/2005 (see also Figure 3.9.3).6

Figure 3.9.2: Pertussis notification rates, Australia, 1993 to 2007,* by age group and year of diagnosis

Figure 3.9.2:  Pertussis notification rates, Australia, 1993 to 2007, by age group and year of diagnosis

* Notifications where the date of diagnosis was between January 1993 and December 2007.

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Figure 3.9.3: Pertussis hospitalisation rates, Australia, 1993/1994 to 2006/2007,* by age group and year of separation

Figure 3.9.3:  Pertussis hospitalisation rates, Australia, 1993/1994 to 2006/2007, by age group and year of separation

* Hospitalisations where the date of separation was between July 1993 and June 2007.

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The lowest notification rates for both years were in the 5–9 years age group. The lowest hospitalisation rates in 2005/2006 and 2006/2007 were in the 5–9 and 10–19 years age groups, respectively.

The 10–19 years age group accounted for 7.4% of all pertussis notifications in 2006–2007 (n=1,211) and 2.7% of all hospitalisations (n=24). The 10–19 years age group had very high notification rates in 2003 (57.6 per 100,000) and 2004 (75.4 per 100,000) but rates started declining in 2005 (41.2 per 100,000) (Figure 3.9.2). In 2006 and 2007, there was a further decline in the notification rate for this age group to 26.3 and 16.6 per 100,000, respectively, with the rate remaining below that of people aged 20–59 years and ≥60 years (Figure 3.9.2). In both 2006 and 2007, the notification rate was higher in the 15–19 years age group than in the 10–14 years age group (34.1 vs 18.4 per 100,000 and 19.2 vs 13.9 per 100,000, respectively). In 2005/2006, the hospitalisation rate was lower in the 15–19 years age group than in the 10–14 years age group (0.1 vs 1.3 per 100,000), while in 2006/2007 it was 0.3 per 100,000 in the 15–19 years age group with no hospitalisations in the 10–14 years age group.

Adults aged 20–59 years, who constitute approximately 56% of the total Australian population, accounted for 60.6% of notifications (n=9,888) and 27.3% of hospitalisations (n=241), with an average annual hospitalisation rate of 1.0 per 100,000 compared with 0.8 per 100,000 for the previous 3 years 2003–2005.6 The notification rates in all adults aged ≥20 years continued to rise in 2006, reaching a record high of 64.4 notifications per 100,000, but dropped substantially to 28.3 notifications per 100,000 in 2007. The proportions of notifications in these age groups have increased from 35%–45% between 1993 and 1998 to 87% in 2006–2007 (Figure 3.9.2). Older persons aged ≥60 years, constituting about 18% of the total population, accounted for 26.4% of notifications (n=4,310) and 29.9% of hospitalisations (n=264). The hospitalisation rate for this age group reached a record high of 3.6 per 100,000 during this reporting period (Figure 3.9.4).

Figure 3.9.4: Pertussis hospitalisation rates, Australia, 1993/1994 to 2006/2007,* by age group (excluding <1 year) and year of separation

Figure 3.9.4:  Pertussis hospitalisation rates, Australia, 1993/1994 to 2006/2007* by age group (excluding less than1 year) and year of separation

* Hospitalisations where the date of separation was between July 1993 and June 2007.

The overall male:female ratio was 1:1.5 for notifications and 1:1.3 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 10–19 years, where the male:female ratio was 1.9:1.

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Geographical distribution

Periodic epidemics of pertussis occur in Australia at intervals of 3–4 years. However, the frequency and length of the epidemic cycles vary between jurisdictions, particularly in the less populated and/or geographically isolated regions, such as the Northern Territory, Tasmania and Western Australia. During the period of review, there was a large variation in notification (Appendix 6.2) and hospitalisation rates (Appendix 6.3) between regions and years. South Australia experienced a pertussis epidemic in 2006 with a notification rate of 138.9 per 100,000 population. The Australian Capital Territory (77.2 per 100,000) and New South Wales (72.1 per 100,000) also experienced elevated notification rates in 2006. However, there was a significant drop in notification rates during 2007 across most states and territories.

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Vaccination status

Completion of the vaccination status field in NNDSS was expected for all pertussis notifications aged <15 years. For the period 2006–2007, 76% of the notifications aged 0–14 years had information about vaccination status. Missing data on vaccination status varied by age group, ranging from 16% for notifications aged 4–9 years, to 0% for those aged 6–11 months. The percentages of notifications fully and partially vaccinated for age were calculated for children aged <9 years with a known vaccination status. The proportion of cases reported to be fully vaccinated for age rose from 42% in infants <6 months of age to 81% in those aged 1–4 years (Table 3.9.2). Within the 0–5 months age group, 5/83 notifications recorded as ‘fully vaccinated for age’ were in infants <2 months of age who were not expected to have received any vaccine doses.

Table 3.9.2: Vaccination status of notified pertussis cases aged 0 months to 9 years, Australia, 2006 to 2007,* by age group

Age group
Vaccination status
(2006–2007)
Fully vaccinated for age Partially vaccinated for age
n % n %
0–5 months
83
42
11
6
6–11 months
50
72
3
4
12–47 months
177
81
4
2
4–9 years
217
68
6
2

* Notifications where the date of diagnosis was between January 2006 and December 2007.

† Percentage of those within the age group with a known vaccination status.

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Comment

Pertussis continues to be one of the most difficult vaccine preventable diseases to control in Australia. Following on from the epidemic period in 2005, the number of cases was again elevated in 2006, with many jurisdictions also having an apparent epidemic. This may be, in part, due to errors in diagnosis as, in October 2006, Australia’s largest distributor of pertussis serology kits announced a major revision in the cut-off for reporting of a positive test. Subsequent revision resulted in a sharp decline in pertussis notifications in the last months of 2006. This led to concern about the validity of all notifications for 2006. However, re-testing of the majority of specimens during this period was not possible, so validation of the notifications could not be performed. These issues with the specificity of serologic diagnosis primarily affected those aged >20 years.

There has been a downward trend in the notification rate among 10–19 year olds since 2002 and a sharper decline in the rate since 2005 which continued during 2006 and 2007. This is likely to reflect the impact of the adult-formulated diphtheria, tetanus and acellular pertussis vaccine (dTpa), recommended in September 2003 and funded since January 2004 as a booster for adolescents, and improved coverage of the preschool age booster dose of DTPa following its replacement of whole-cell pertussis vaccine. The lower notification rate in 2006–2007 for the 10–14 years age group compared with the 15–19 years age group is probably due to the eligibility for a DTPa preschool booster in the former cohort group. Interpretation of the impact of the adolescent booster within these smaller age groups is complicated by differing delivery among states and territories and over time. As Australian school-based dTpa programs mature, and successive cohorts are vaccinated in future years, pertussis in adolescents would be expected to become well controlled, as occurred in 5–9 year olds following the introduction of the preschool booster.

In this reporting period, pertussis was primarily a problem in two broad age groups: first, infants who had the highest notification and hospitalisation rates, particularly those <4 months of age who are too young to have received 2 or more doses of DTPa; and second, people aged ≥20 years, who account for the majority of pertussis notifications, at least in part related to the increased use of serological testing to diagnose pertussis in this age group,7 and increased diagnostic awareness of pertussis in adults. Hospitalisations in adults are most likely to be related to complications. Although severe morbidity and mortality are less likely, adults are a significant reservoir of infection and increased circulation of pertussis can facilitate transmission to susceptible infants who are too young to be vaccinated.8–11

The recent increase in the incidence and burden of pertussis notifications in persons aged ≥60 years warrants further investigation. As with parents, grandparents are an important source of pertussis transmission to infants.8 It is also unclear whether the morbidity of pertussis in older people is greater, or if complications are more likely to occur, but it is of note that 4 of 11 pertussis deaths in the past 7 years have been recorded in people aged ≥80 years.6

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References

1. Pertussis and parapertussis. In: Heymann DL, ed. Control of Communicable Diseases Manual. 19th edn. Washington, DC: American Public Health Association, 2008.

2. 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 24 August 2009.

3. McIntyre P, Amin J, Gidding H, Hull B, Torvaldsen S, Tucker A, et al. Vaccine Preventable Diseases and Vaccination Coverage in Australia, 1993–1998. Commun Dis Intell 2000;24(Suppl):S1–S83.

4. McIntyre P, Gidding H, Gilmour R, Lawrence G, Hull B, Horby P, et al. Vaccine Preventable Diseases and Vaccination Coverage in Australia, 1999 to 2000. Commun Dis Intell 2002;26(Suppl):S1–S111.

5. Brotherton J, McIntyre P, Puech M, Wang H, Gidding H, Hull B, et al. Vaccine Preventable Diseases and Vaccination Coverage in Australia, 2001 to 2002. Commun Dis Intell 2004;28(Suppl 2):S1–S116.

6. Brotherton J, Wang H, Schaffer A, Quinn H, Menzies R, Hull B, et al. Vaccine Preventable Diseases and Vaccination Coverage in Australia, 2003 to 2005. Commun Dis Intell 2007;31(Suppl):S1–S152.

7. Quinn HE, McIntyre PB. Pertussis epidemiology in Australia over the decade 1995–2005 – trends by region and age group. Commun Dis Intell 2007;31(2):205–215.

8. Bisgard KM, Pascual FB, Ehresmann KR, Miller CA, Cianfrini C, Jennings CE, et al. Infant pertussis: who was the source? Pediatr Infect Dis J 2004;23(11):985–989.

9. Elliott E, McIntyre P, Ridley G, Morris A, Massie J, McEniery J, et al. National study of infants hospitalized with pertussis in the acellular vaccine era. Pediatr Infect Dis J 2004;23(3):246–252.

10. Schellekens J, Wirsing von König CH, Gardner P. Pertussis sources of infection and routes of transmission in the vaccination era. Pediatr Infect Dis J 2005;24(5 Suppl):S19–S24.

11. Wendelboe AM, Njamkepo E, Bourillon A, Floret DD, Gaudelus J, Gerber M, et al. Transmission of Bordetella pertussis to young infants. Pediatr Infect Dis J 2007;26(4):293–299.

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