Pertussis epidemiology in adults: Retrospective analysis of pertussis incidence and association with comorbidities among adult populations in Aotearoa New Zealand, using national administrative datasets

Background: In New Zealand, approximately half reported pertussis cases are adult. Studies indicate underestimated pertussis burden in this population and probable reservoir for childhood pertussis. Pertussis is linked to chronic obstructive pulmonary disease (COPD) development and increased risk with pre-existing COPD. While acellular pertussis vaccines are available for adults, data on pertussis disease burden in adults and association with COPD remain limited. Aim: To estimate pertussis incidence in New Zealand adult health service user (HSU) population aged >= 18 between 2008-2019 and inform adult pertussis vaccination strategies by assessing disease burden and risk factors in different adult populations. Methods: Retrospective observational cohort study using an HSU cohort, formed by linking administrative health data using unique National Health Index identifier. For primary analysis, annual incidence rates were calculated using pertussis hospitalisations and notifications. In secondary analysis, Cox proportional hazards survival analyses explored association between pertussis in adults and chronic comorbidities. Results: The cohort had 2,907,258 participants in 2008 and grew to 3,513,327 by 2019, with 11,139 pertussis cases reported. Highest annual incidence rate of 84.77 per 100,000 PYRS in 2012, notably affecting females, those aged 30-49 years, and European or Ma<overline>ori ethnicity. Adjusting for sociodemographic variables found no significant risk of prior pertussis notification leading to comorbidity diagnosis (Adjusted-HR: 0.972). However, individuals with prior comorbidity diagnosis had 16% greater risk of receiving pertussis notification or diagnosis (Adjusted-HR: 1.162). Conclusions: Study found significant pertussis burden among the HSU adult cohort and highlighted higher risk of pertussis for those with recent comorbidity diagnoses. Vaccination for pertussis should be recommended for individuals with comorbidities to reduce infection risk and disease severity. GPs must have capability to test for pertussis, given it is notifiable disease with implications for individuals, their families, and broader population. High-quality disease surveillance is crucial for informing policy decisions.