Hospitalisations due to pertussis in New Zealand in the pre‐immunisation and mass immunisation eras

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Abstract
Aim: Pertussis disease burden in New Zealand in recent decades has been large compared with other developed countries. However, these comparisons use data from relatively short time periods given the long epidemic cycle of pertussis. To better understand the current disease burden, this study examined pertussis hospitalisation data in New Zealand in both the pre-immunisation and mass immunisation eras. Methods: Hospital discharge data and population data from 1873 to 2004 were used to estimate average pertussis hospital discharge rates per decade. Rates were compared using relative risks and 95% confidence intervals (CI). Results: Average annual pertussis hospitalisation rates per 100 000 were less than two from 1873 to 1919, increased to 12 in the 1940s, decreased to less than four in the 1960s and have increased since then with the rate in the current decade being 5.8. Compared with the 1960s (3.8 per 100 000) the average annual rate has been significantly greater in the 1980s (RR = 1.11, 95% CI 1.03, 1.21), 1990s (RR = 1.33, 95% CI 1.23, 1.44) and 2000s (RR = 1.55, 95% CI 1.42, 1.68). Since 1960 hospitalisation rates have increased for those less than one year old, one to four years old and five years and older. The increases have been most marked for infants (RR 2000s vs. 1960s = 2.87, 95% CI 2.59, 3.18). Conclusion: After an initial decline following mass immunisation, pertussis hospitalisation rates in New Zealand have subsequently increased steadily. To reduce pertussis disease burden improved immunisation coverage and timeliness is required and consideration given to spreading the pertussis vaccine schedule over a wider age range.