The investigators intend to observe the inflammatory marker procalcitonin (PCT) in comparison with patient variables on morbidity, mortality and antibiotic treatment.

The overall aim is to observe if a low PCT value can have potential in reducing unessesary antibiotic use in seasonal influenza patients.

This is an observational study of the clinical utility of early PCT measurements in seasonal influenza, with particular attention to PCT's predictive value for prognosis and bacterial respiratory superinfection. The aim is to evaluate PCT as a supplementary aid in the standard clinical evaluation of influenza patients. Patients may be included if a positive influenza test is reported within 48 hours after an initial clinical suspicion of influenza, and the hospital stay is lasting more than one day. The investigators have included 74 patients from the second half of the 2018-19 influenza season and will continue to include patients from the complete 2019-20 season.

Primary endpoints: Mortality (in-hospital and within 30 days post-discharge), bacterial respiratory tract superinfection.

Secondary endpoints: Intensive care unit stay, the proportion of patients on antibiotic use and total antibiotic use in doses and LOT ("length-of-treatment").

Inclusion Criteria:

• patient admitted to the hospital with positive influenza test

Exclusion Criteria:

• none

seasonal influenza patients, adult population