OBJECTIVES: The burden of influenza varies across time and is affected by variations in circulating strains, population immunity, vaccination coverage and other risk factors including comorbidities and age. This study explored the impact of the presence of comorbidities on influenza-related hospitalisation costs and length of stay (LOS) in adults.

METHODS: The study analysed four years of pre-COVID HES data (September 2016 – March 2020) and extracted adult patients with an influenza diagnosis (ICD-10 code J09-J11 in the primary or secondary location) in a day case or inpatient setting, identifying all patients with primary or secondary diagnosis related to any relevant comorbidity in the influenza spell. Hospitalisation costs and LOS were analysed by age, gender, and presence of comorbidities. Multi-level regression models were run on hospitalisation LOS and costs to ensure estimates captured any within-patient effects and adjusted for age, gender and other comorbid conditions.

RESULTS: In total, 119,495 patients were hospitalised with influenza generating a total cost of £401m, an average of £3,159 per spell. The average spell LOS and cost increased with age and the presence of comorbidities. There was a 4-day difference in median length of stay between those with and without comorbidities (6 and 2 days, respectively). Those with comorbidities recorded average costs of £3,569 compared to £1,458 for those without. Chronic heart disease was the most common comorbid condition and increased average cost even further to £4,397. Presence of comorbidities was significantly associated with hospitalisation cost; the cost for a patient with comorbidities was 214% of the cost for a patient without [95% CI 208-221%, p<0.01].

CONCLUSIONS: The study demonstrates the effect of comorbid conditions on influenza-related hospitalisation costs and length of stay (LOS) in adults. It strengthens the value of annual immunization for those in at-risk clinical groups in order to reduce the clinical and economic burden.