The Hospital Burden of Flu in Italy: a retrospective study on administrative data from season 2014–2015 to 2018–2019

Background Every year in Italy, influenza affects about 4 million people.

Almost 5% of them are hospitalised.

During peak illness, enormous pressure is placed on healthcare and economic systems.

This study aims to quantify the clinical and economic burden of severe influenza during 5 epidemic seasons (2014–2019) from administrative claims data.

Methods Patients hospitalized with a diagnosis of influenza between October 2014, and April 2019, were analyzed.

Clinical characteristics and administrative information were retrieved from health-related Administrative Databases (ADs) of 4 Italian Local Health Units (LHUs).

The date of first admission was set as the Index Date (ID).

A follow-up period of six months after ID was considered to account for complications and re-hospitalizations, while a lookback period (2 years before ID) was set to assess the prevalence of underlying comorbidities.

Results Out of 2,333 patients with severe influenza, 44.1% were adults ≥ 65, and 25.6% young individuals aged 0–17.

46.8% had comorbidities (i.e., were at risk), mainly cardiovascular and metabolic diseases (45.3%), and chronic conditions (24.7%).

The highest hospitalization rates were among the elderly (≥ 75) and the young individuals (0–17), and were 37.6 and 19.5/100,000 inhabitants/year, respectively.

The average hospital stay was 8 days (IQR: 14 − 4).

It was higher for older individuals (≥ 65 years, 11 days, [17 − 6]) and for those with comorbidities (9 days, [16 − 6]), p-value < 0.001.

Similarly, mortality was higher in elderly and those at risk (p-value < 0.001).

Respiratory complications occurred in 12.7% of patients, and cardiovascular disorders in 5.9%.

Total influenza-related costs were €9.7 million with hospitalization accounting for 95% of them.

47.3% of hospitalization costs were associated with individuals ≥ 65 and 52.9% with patients at risk.

The average hospitalisation cost per patient was € 4,007.

Conclusions This retrospective study showed that during the 2014–2019 influenza seasons in Italy, individuals of extreme ages and those with pre-existing medical conditions, were more likely to be hospitalized with severe influenza.

Together with complications and ageing, they worsen patient’s outcome and may lead to a prolonged hospitalization, thus increasing healthcare utilization and costs.

Our data generate real-world evidence on the burden of influenza, useful to inform public health decision-making.