The 1996 pertussis epidemic in New Zealand: descriptive epidemiology

To describe the 1996 pertussis epidemic.

Hospitalisation, notification and laboratory data were used to describe the 1996 pertussis epidemic and compare it with previous epidemics.

The 1996 epidemic spanned 24 months. The crude hospitalisation rate from 1 June 1995 to 31 May 1997, was 10.1 per 100,000 person years, being highest for children aged six weeks to two months (42 to 90 days old inclusive; 1402 per 100,000). The 1996 epidemic involved more hospitalisations than the 1991 and 1986 epidemics, and a greater proportion for children under the age of one year (77%), compared to previous epidemics (60-70%). There were no deaths. Pertussis only became notifiable from 1 June 1996. The crude notification rate for the following twelve months was 19.8 per 100,000 (equivalent hospitalisation rate 6.7 per 100,000); children aged six weeks to two months of age had the highest notification rate (531 per 100,000; equivalent hospitalisation rate 1021 per 100,000). In 1996-97, children aged under 15 months accounted for 21% of notifications, but 82% of hospitalisations. Europeans tended to have higher rates of notifications than non-Europeans, but lower rates of hospitalisation.

New Zealand continues to experience high rates of pertussis as a result of inadequate immunisation coverage. The increase in hospitalisations during the 1996 epidemic may reflect a real increase in the population-based incidence, or other changes (e.g. hospitalisation practice, increase in vulnerable children with poor access to primary care). Improved rates, accuracy and completeness of pertussis notifications will improve the ability of notification data to accurately describe future epidemics and estimate vaccine effectiveness. Further debate is required regarding the aims of pertussis immunisation; accelerating the timing of the first three doses and adding further doses of pertussis vaccine on the national immunisation schedule; and the role of acellular pertussis vaccines. In the meantime, the priority must be increasing on-time immunisation coverage.