Risk of Severe Influenza among Adults with Chronic Medical Conditions.

BACKGROUND: Severe influenza illness is presumed more common in adults with chronic medical conditions (CMC), but evidence is sparse and often combined into broad CMC categories.

METHODS: Residents (aged 18-80 years) of Central and South Auckland hospitalized for WHO-defined severe acute respiratory illness (SARI) (2012-2015) underwent influenza virus PCR testing. CMC statuses for Auckland residents were modelled using hospitalization ICD-10 codes, pharmaceutical claims, and laboratory results. Population-level influenza rates in adults with congestive heart failure (CHF), coronary artery disease (CAD), cerebrovascular accidents (CVA), chronic obstructive pulmonary disease (COPD), asthma, diabetes mellitus (DM), and end-stage renal disease (ESRD) were calculated by Poisson regression stratified by age and adjusted for ethnicity.

RESULTS: Among 891,276 adults, 2,435 influenza-associated SARI hospitalizations occurred. Rates were significantly higher in those with CMCs compared with those without the respective CMC except older adults with DM or those aged <65 years with CVA. The largest effects occurred with CHF (Incidence Rate Ratio [IRR] range: 4.84-13.4 across age strata), ESRD (IRR range: 3.30-9.02), CAD (IRR range= 2.77-10.7), and COPD (IRR range: 5.89-8.78) and tapered with age.

CONCLUSIONS: Our findings support the increased risk of severe, laboratory-confirmed influenza disease among adults with specific CMCs compared those without these conditions.