Population-Based Study of Pertussis Incidence and Risk Factors among Persons >50 Years of Age, Australia

Abstract

Despite vaccination programs, pertussis has been poorly controlled, especially among older adults in Australia. This longitudinal, retrospective, observational study aimed to estimate the incidence and risk factors of pertussis among persons ≥50 years of age in Australia in the primary care setting, including those with underlying chronic obstructive pulmonary disease (COPD) or asthma. We used the IQVIA general practitioner electronic medical record database to identify patients ≥50 years of age with a clinical diagnosis of pertussis during 2015-2019. Pertussis incidence rates ranged from 57.6 to 91.4 per 100,000 persons and were higher among women and highest in those 50-64 years of age. Patients with COPD or asthma had higher incidence rates and an increased risk for pertussis compared with the overall population ≥50 years of age. Our findings suggest that persons ≥50 years of age in Australia with COPD or asthma have a higher incidence of and risk for pertussis diagnosis.

Keywords: Australia; COPD; Pertussis; asthma; bacteria; elderly; incidence; respiratory infections; risk factors; vaccination; vaccine-preventable diseases; whooping cough.

Figure 1

Identification of pertussis cases in population-based study of pertussis incidence and risk factors among persons >50 years of age, Australia. A) Overall study population; B) patients with COPD/asthma. *Differentiation between confirmed, probable, and suspected cases was not possible because lab testing is not routinely being performed in all patients and cases were identified from the GP EMR based on the diagnosis label. COPD, chronic obstructive pulmonary disease; EMR, electronic medical record; GP, general practitioner.

Figure 2

National notification rates compared with annual incidence of pertussis in population-based study of pertussis incidence and risk factors among persons >50 years of age, Australia. A) Overall study population; B) by age group: group 1, 50–64 y; group 2, 65–74 y; group 3, 75–84 y; group 4, ≥85 y; C) by sex. Incidence rates are reported per 100,000 persons; error bars indicate 95% CIs. Both NNDSS and GP EMR data consist of persons ≥50 years of age. Data in 2015 were projected to 12-month period because a run-in period/landmark was applied to rule out prevalent pertussis cases carried forward from the previous year. Data for 2016–2019 are as observed. EMR, electronic medical records; GP, general practitioner; NNDSS, National Notifiable Diseases Surveillance System.

Figure 3

Annual incidence of pertussis among persons >50 years of age with and without COPD in population-based study of pertussis incidence and risk factors, Australia. A) Overall study population; B) by age group: group 1, 50–64 y; group 2, 65–74 y; group 3, 75–84 y; group 4, ≥85 y; C) by sex. Incidence rates are reported per 100,000 persons; error bars indicate 95% CIs. COPD cases were defined based on diagnosis label or prescription of reliever/corticosteroid inhaler (≥1 refill of the same product). Data in 2015 were projected to 12-month period because a run-in period/landmark was applied to rule out prevalent pertussis cases carried forward from the previous year. Data for 2016–2019 are as observed. COPD, chronic obstructive pulmonary disease.

Figure 4

Annual incidence of pertussis among persons >50 years of age with and without asthma in population-based study of pertussis incidence and risk factors, Australia. A) Overall study population; B) by age group: group 1, 50–64 y; group 2, 65–74 y; group 3, 75–84 y; group 4, ≥85 y; C) by sex. Incidence rates are reported per 100,000 persons; error bars indicate 95% CIs. Asthma cases were defined based on diagnosis label or prescription of reliever/corticosteroid inhaler (≥1 refill of the same product). Data in 2015 were projected to 12-month period because a run-in period/landmark was applied to rule out prevalent pertussis cases carried forward from the previous year. Data for 2016–2019 are as observed.

Figure 5

Comparison of the monthly number of pertussis cases reported to the NNDSS and identified in the primary and sensitivity analyses of population-based study of pertussis incidence and risk factors among persons >50 years of age, Australia. A) Cases captured by the NNDSS; B) cases in the primary overall study population; C) cases in the sensitivity analysis cohort. NNDSS, National Notifiable Diseases Surveillance System.