Pertussis (whooping cough) remains an important public health problem in Australia. Although mortality and morbidity from pertussis declined dramatically following the introduction of mass vaccination programs in 1953, the level of morbidity remains unacceptably high for a vaccine-preventable disease. Aims and methods The primary aims of this thesis were (i) to ascertain the epidemiology of pertussis in Australia between 1993 and 2000 by analysing and interpreting sources of routinely collected data on pertussis; and (ii) to examine the effectiveness of vaccination against pertussis in a number of ways. Data from three primary national sources (notifications of disease, hospitalisations for pertussis and death certificates) were used to examine the burden from pertussis in Australia over these eight years. Analyses included the age distribution of cases, temporal and geographic trends, comparisons of notification and hospitalisation data, and the impact of differences in the method of diagnosis of notified cases between years and age groups. In addition to analyses at the national level using data from the national databases, further detailed analyses were undertaken at the State level for New South Wales (NSW), the most populous Australian State. Pertussis vaccine coverage was estimated using data from the recently established Australian Childhood Immunisation Register (ACIR); these data were also used to track the transition from whole-cell to acellular pertussis vaccines. The different types of studies used to evaluate vaccine effectiveness were reviewed, and a method suitable for ongoing estimation of vaccine effectiveness in Australia was developed. This was then applied to the NSW data, to determine the effectiveness of pertussis vaccination in this State. Main findings The annual notification rate for pertussis in Australia ranged from 23�59 per 100 000 population over the eight years. Infants had the highest notification and hospitalisation rates in Australia � they accounted for 5percent of notifications, 61percent of hospitalisations and 100percent of deaths. Age-specific notification and hospitalisation rates in children aged less than two years strongly suggested a protective effect of vaccination, with the greatest reduction in rate coinciding with eligibility to receive a second dose of pertussis vaccine at four months of age. Notification rates among 5�9 year olds progressively decreased in successive age cohorts, consistent with an effect of the introduction in 1994 of a pertussis vaccine booster for preschool-aged children. Although adults (persons aged 15 years or more) accounted for half the notifications, they had the lowest notification rate. The highest numbers of pertussis notifications were in 1997, when most jurisdictions experienced an epidemic. Notification and hospitalisation rates varied across the States and Territories and also across smaller geographic regions in NSW. Areas and years with high notification rates tended to also have high hospitalisation rates, suggesting that trends in notifications reflected trends in incidence. The number of infant hospitalisations in NSW between July 1993 and June 1999 exceeded the number of notifications by 32percent, highlighting the extent of under-notification. Overall, and particularly amongst those aged more than 12 months, the majority of cases notified in NSW were based on the results of serological tests. The proportion diagnosed by culture of the organism was greatest in infants; the proportion diagnosed by serological tests increased with age. There was no evidence that the use of serology had increased since 1994 in NSW, hence changes in notification rates after this time are unlikely to be attributable to increased use of serological diagnosis. ACIR records indicated that in December 2000, 92percent of one-year-old children had received three doses of diphtheria-tetanus-pertussis (DTP) vaccine and 90percent of two-year-olds had received four doses. Vaccine coverage varied by jurisdiction. Since 1997, there was an increased use of DTP vaccines containing acellular pertussis components with a corresponding decrease in the use of vaccines containing whole-cell components. In 2000, almost all DTP vaccines administered contained acellular pertussis components. The results of the vaccine effectiveness study showed that pertussis vaccination was highly effective at preventing pertussis in NSW children, as measured by notified cases. Vaccine effectiveness was highest (91percent) in the youngest age group (8�23 months) and lowest (78percent) in the oldest age group (9�13 years). The screening method has not previously been used to estimate pertussis vaccine effectiveness in Australia. Conclusions This thesis demonstrates the value of integrating varied data sources in estimating the disease burden from pertussis. The data presented here show that the disease burden is substantial in all age groups, despite high levels of vaccine coverage in infants and children. This problem of disease control does not appear to be due to lack of vaccine effectiveness, but there is evidence of waning immunity over time. The analyses presented here form a basis for the ongoing monitoring of trends in pertussis epidemiology following the replacement of whole-cell by acellular pertussis vaccines, and will assist consideration of the need for additional booster doses in adolescents and adults.
| Identifer | oai:union.ndltd.org:ADTP/283166 |
| Date | January 2001 |
| Creators | Torvaldsen, Siranda |
| Publisher | University of Sydney. Paediatrics and Child Health |
| Source Sets | Australiasian Digital Theses Program |
| Language | English, en_AU |
| Detected Language | English |
| Rights | Copyright Torvaldsen, Siranda;http://www.library.usyd.edu.au/copyright.html |
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