Clinical and economic burden of physician-diagnosed influenza in adults during the 2017/2018 epidemic season in Spain

BACKGROUND: Influenza is an acutely debilitating respiratory infection, contributing significantly to outpatient visits and hospitalizations. Spain lacks comprehensive and updated data on the burden of influenza, particularly in the outpatient setting. Our study aimed to fill this gap by estimating the clinical and economic burden of physician-diagnosed influenza cases in adults from four Spanish regions, stratified by age groups and presence of comorbidities. METHODS: A retrospective cost-of-illness study was conducted using data from an electronic medical records database from the National Healthcare Service (NHS) of four Spanish regions for individuals aged ≥ 18 years diagnosed for influenza during the 2017/2018 epidemic season. Health resource utilization and related cost data were collected, including primary care visits, referrals to other specialists, visits to the emergency department, hospitalizations, and prescribed medicines. RESULTS: The study reported a total of 28,381 patients aged ≥ 18 years diagnosed with influenza, corresponding to 1,804 cases per 100,000 population. Most patients were aged < 65 years: 60.5% (n = 17,166) aged 18–49 and 26.3% (n = 7,451) 50–64 years. A total of 39.2% (n = 11,132) of patients presented a comorbidity. Cardiovascular diseases were the most common comorbidity reported along with influenza. The mean healthcare cost per case was estimated at €235.1 in population aged 18–49 years, increasing by 1.7 and 4.9 times in those aged 50–64 (€402.0) and ≥ 65 (€1,149.0), respectively. The mean healthcare cost per case was 3.2 times higher in patients with comorbidities. The total healthcare cost of medically attended influenza cases was mainly driven by primary care (45.1%) and hospitalization (42.0%). Patients aged 18–64 years old accounted for 61.9% of the costs of medically attended influenza. Irrespective of age, patients with comorbidities accounted for 67.1% of costs. CONCLUSIONS: Season 2017/2018 was associated with a considerable burden of influenza in Spain, which increased with age and presence of comorbidities. Individuals with comorbidities accounted for most of the costs of influenza. Results suggest that population aged 18–64 years old is generating the highest share of costs to the NHS when all healthcare costs are considered. Preventive strategies targeting subjects with comorbidities, regardless of age, should be warranted. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-022-14732-2.