The outbreak that had to happen: Bordetella pertussis in north-west Western Australia in 1999.

In late 1999, an outbreak of Bordetella pertussis occurred in a small town in North-West Western Australia. We undertook an investigation to describe the outbreak and to identify strategies to minimise the impact of future pertussis outbreaks in Australia. In November, people with respiratory symptoms were reviewed in an emergency pertussis clinic, which provided antibiotic treatment or prophylaxis. We conducted a school survey to enhance case ascertainment and followed up those attending the clinic by telephone. Fifty-nine cases of confirmed or probable B. pertussis infection were identified from 124 households (482 persons). Ages ranged from 5 months to 67 years, with children aged 9 to 11 years comprising 24 cases (41%). Early missed diagnoses and a school camp in September attended by 2 symptomatic children appeared to facilitate spread of infection, with the outbreak peak occurring in November. From immunisation records, childhood vaccine coverage in this sample was estimated at 96 per cent. All 21 cases of pertussis among the group under 10 years of age were at least partially vaccinated. There was only one laboratory confirmed case in the high-risk, under one-year of age category. Even in highly immunised populations periodic pertussis outbreaks are inevitable reflecting a vaccine efficacy of about 80 per cent and waning immunity with increasing age. Prevention of pertussis outbreaks depends not only on high vaccination coverage among young children but also early diagnosis and management of cases and their contacts. Clinicians should consider pertussis in the differential diagnosis of persistent cough illness in people of all ages--even those previously immunised.