Pertussis in older adults: prospective study of risk factors and morbidity.

BACKGROUND: There is limited information on the incidence, morbidity and risk factors for pertussis in adults, particularly those aged over 65 years.

METHODS: Population-based prospective cohort study of 263094 adults aged over 45 years (mean 62.8 years) recruited in the Australian state of New South Wales (the 45 and Up Study) between 2006 and 2008, and followed by record-linkage to laboratory-confirmed pertussis notifications, hospitalizations, and death records. The incidence of pertussis notifications and hospitalizations and relative risk (RR) of pertussis according to various participant characteristics was estimated using proportional hazards models.

RESULTS: Over a total follow-up of 217524 person-years, 205 adults had a pertussis notification and 12 were hospitalized; the incidence rate was 94 (95% confidence interval [CI], 82-108) and 5.5 (95% CI, 3.1-9.7) per 100000 person-years, respectively. The incidence of a pertussis notification did not differ by age but hospitalization rates progressively increased (2.2, 8.5, and 13.5 per 100000 person-years in age groups 45-64, 65-74, and 75+ years, respectively; P(trend) = .01). After adjusting for age, sex, and other factors, adults with a high body mass index (BMI; RR=1.52; 95% CI, 1.06-2.19 for BMI 30+kg/m(2) vs BMI <25 kg/m(2)) and with preexisting asthma (RR=1.64; 95% CI, 1.06-2.55 compared to those without asthma) were more likely to be notified.

CONCLUSIONS: Adults older than 65 years are more likely to be hospitalized for pertussis than those aged 45-64 years. Obesity and preexisting asthma were associated with a higher likelihood of pertussis notification. These findings suggest that pertussis vaccination would be particularly important for adults with these characteristics.