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The pertussis epidemic experienced in NSW in 2008–2009 was likely to be in part due to changes in diagnostic practice since 2007, which amplifieddiseasenotifications.Weusedpopulationbased seroepidemiology as a less biased means of interpreting age-specific pertussis infection patterns inNSWfrom three serosurveys undertaken in 1997–98 (duringanepidemic), 2002 (post-epidemic) and 2007 (inter-epidemic), using a standardised pertussis toxin IgG enzyme-linked immunosorbent assay (ELISA). There was a decrease in the proportion of high anti-pertussis toxin IgG titres (.62.5 ELISAUnits/mL) across all age groups in the 2007 serosurvey compared to the previous two serosurveys. In the 2007 serosurvey, the proportion of undetectable (,5ELISAUnits/mL) anti-pertussis toxin IgG titres increased in many age groups. The seroepidemiological profiles of the three serosurveys demonstrate fluctuating immunity profiles related to changes in vaccination schedules. Despite longstanding immunisation programs in developed countries such as Australia and the United States, periodic epidemics of pertussis continue at intervals of 3–4 years on a background of endemic circulation. In 2008–2009, New South Wales (NSW) experienced a sustained pertussis epidemic which was unusually large in magnitude. However, interpretation of notification data from the current pertussis epidemic is confounded by changes in diagnostic practice since 2007. The shift to widespread use of polymerase chain reaction for pertussis diagnosis in all age groups is likely to have amplified the number of cases notified during the epidemic, particularly in children. Population-based, cross-sectional seroepidemiology offers a less biased means of comparison of age-specific patterns of pertussis compared to other disease surveillance methods, but requires acceptable standardisation and reproducibility of serologic tests and the ability to extrapolate these results to estimates of symptomatic cases. The European Sero-Epidemiology Network (ESEN) standardised the use of a serologic criterion for recent infection, measured in IgG-pertussis toxin (PT) using enzyme-linked immunosorbent assay (ELISA) Units (EU) between participating laboratories using different serologic methods. This development allowed meaningful comparison of Bordetella pertussis seroepidemiology in six European countries. This standardised methodology has previously been applied to Australian sera collected in 1997–98 and 2002 to describe national pertussis trends by age and over time. Since then, several important changes in pertussis vaccine schedules have occurred in all states and territories, including NSW. First, the 18-month booster dose of diphtheriatetanus-acellular pertussis vaccine (DTPa) was removed from theNational Immunisation Program schedule in 2003. Second, an adult formulation acellular pertussis-containing vaccine (dTpa) was added to the schedule for adolescents, with school-based vaccination commencing in May 2004. Earlier schedule changes are outlined in Table 1. This study compares NSW data from three cross-sectional serosurveys, undertaken in 1997–98 (during an epidemic), 2002 (post-epidemic) and 2007 (inter-epidemic). The aim was to evaluate age-specific patterns of presumptive recent pertussis infection in the context of changes to the vaccine schedule and pertussis notifications. Methods Population and study design The 2007 sera used in this study were selected from a bank of approximately 7200 sera collected opportunistically 224 | Vol. 22(11–12) 2011 NSW Public Health Bulletin 10.1071/NB11023 from a geographically representative group of 29 diagnostic laboratories receiving samples from hospitalised and ambulant persons throughout Australia as part of a national serosurveillance program, but was restricted to NSW laboratories for this study. The sera in the opportunistic sample were residual from specimens submitted for diagnostic testing and would otherwise have been discarded. Residual sera were from subjects who: were immunosuppressed; had received multiple or recent (within 3 months) blood transfusions; or were known to have HIV infection, were excluded by staff at the diagnostic laboratory. Sera were identified by amedical record number, sex, age, state/ territory of origin and a unique identifier, to ensure that only one sample from any subject was tested. Approval for the serosurvey was obtained from the Western Sydney Area Health Service Human Research Ethics Committee. In all relevant age groups the required sample size was calculated to achieve a 95% probability of precision 7% around a point estimate based on the expected level of seroprevalence. There were equal numbers of males and females within each age group. The population and study design for the 1997–98 and 2002 serosurveys have been described previously. Across the three serosurveys the proportion of samples from outer regional and remote locations remained similar. In contrast, the proportion of samples from a metropolitan area decreased (85% in 1997–98 to 70% in 2007) and the proportion from an inner regional area increased (10% in 1997–98 to 25% in 2007). Testing and serologic criteria for recent pertussis infection For the 2007 serosurvey a total of 1152 sera randomly selected from those available in each age group were tested using an established ELISA method adapted from Giammanco et al and previously described in full. The 1997–98 serosurvey was conducted at the ESEN reference laboratory at the University of Palermo, Italy and both the 2002 and 2007 analyses were performed at the Centre for Infectious Diseases and Microbiology, Sydney, Australia. The method used in our laboratory was validated against a panel of sera from the ESEN reference laboratory. In 2007 the assay was again validated, using a panel of samples from the 2002 serosurvey. Theminimum level of detection of the assay, defined as the minimum amount of antibody that must be present for the serum to have at least one optical density (OD) value within the linear range of the reference serum response curve, was estimated to be 2 EU/mL. Anti-PT IgG levels were divided into four categories, previously described by the ESEN study group as suggestive of pertussis exposure within certain time periods: ,5EU/mL (undetectable), 5 , 62.5EU/mL (exposure more than 12 months previously), 62.5 , 125EU/mL (exposure within 12 months) and $125EU/mL (exposure within 6 months). Exposure implies ‘significant’ exposure, with antibody response and is inclusive of infection and immunisation. These categories were originally derived by de Melker and colleagues from a longitudinal cohort study of Dutch patients with clinically confirmed pertussis infection, and demonstrated that high anti-PT IgG levels can be a sensitive and specific indicator of recent infection. These levels fell below the nominated threshold levels in almost all patients within 12 months of infection. Statistical analysis Proportions and 95% confidence intervals were calculated for sera in each of the categories described above, by age group. A chi-square test was used to compare proportions by age group and serosurvey, with p values ,0.05 Table 1. Significant pertussis vaccine schedule changes in NSW, 1975]2003 Year Vaccine type Event 1975 DTPw Diphtheria-tetanus-whole cell pertussis (DTPw) national vaccination schedule recommended for infants aged 3, 4 and 5 months DTPw Booster dose for infants aged 15–18 months introduced 1978 DTPw Booster dose removed from schedule 1982 DTPw National vaccination schedule changed to a primary series at 2, 4 and 6 months of age 1985 DTPw Booster dose re-introduced at 18 months of age due to an increase in pertussis incidence in children aged 4–5 years 1994 DTPw Booster dose at 4–5 years of age added to the recommend vaccination schedule 1997 DTPa Diphtheria-tetanus-acellular pertussis (DTPa) recommended for the booster doses

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