Incidence of pertussis

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.

More info on http://www.healthconnect.gov.au/internet/main/publishing.nsf/Content/cda-cdi31suppl.htm~cda-cdi31suppl-3.htm~cda-cdi31suppl-3i.htm

-Satwik