The seroepidemiology of pertussis in Australia during an epidemic period

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Abstract
Studying the epidemiology of pertussis and impact of differing vaccine schedules is difficult because of differing methods of case ascertainment. The advent of internationally standardized serological diagnosis for recent infection has allowed comparison of age-specific pertussis infection among European countries and was applied in Australia at the time of a major national epidemic. In 1997 and 1998, a nationally representative serum bank using residual sera from diagnostic laboratories was established. Measurement of pertussis toxin (PT) IgG level was conducted by a reference laboratory using an enzyme-linked immunosorbent assay standardized for a number of European countries. A titre of 125 EU/ml was interpreted as indicative of recent pertussis infection. The serological data were correlated with age, gender, region and disease epidemiology in Australia. The highest prevalence of recent pertussis infection was in the 5–9 years age group, and the lowest in 1–4 and 25–64 years age groups. In the 5–14 years age group, 29·7% (5–9 years) and 14·6% (10–14 years) of the sample had serological evidence of recent infection, correlating with the pattern of epidemic notifications. The 15- to 24-year-olds had similar high titres but the same notification rate as 25- to 44-year-olds, suggesting ascertainment bias may result in under-notification in the former age group. The prevalence of high titres observed was up to 20-fold higher than some European countries during a similar time period. Although vaccination has reduced the transmission of pertussis in the youngest and most vulnerable age group, pertussis is still endemic in Australia, particularly in older children and the elderly. The Australian vaccination schedule has been changed in an attempt to address this problem, by spacing doses more widely, with the fifth dose at 15–17 years of age. Seroepidemiology for pertussis offers the potential to compare patterns of pertussis between countries and examine the impact of vaccine schedule changes independent of notification and diagnostic bias.