Inclusion Criteria:

• Hospitalized
• Adults 18 year or older
• Moderately severe disease (CRB65 ≤ 2 at time of inclusion)
• Nasopharyngeal swab positive for influenza virus, parainfluenza virus, respiratory syncytial virus (RSV) or human metapneumovirus (hMPV)
• On antibiotic therapy as instituted by the receiving physician from the emergency department
• Signed informed consent must be obtained and documented according to ICH GCP, and national/local regulations.

Exclusion Criteria:

• Requiring ICU admission at screening
• Requiring high-flow oxygen therapy or non-invasive ventilation at screening
• Signs of severe pneumonia (abscesses, massive pleural effusion, a well-defined lobar infiltrate on chest X-ray strongly suggestive of bacterial etiology)
• Not immunocompetent (i.e. on active chemotherapy, corticosteroid therapy equaling ≥ 20 mg prednisolone daily for ≥ 4 weeks, chronic immunosuppression due to solid organ transplant)
• SARS-CoV-2 positive
• Bacteremia
• Urine antigen test positive for legionella
• Any other infection necessitating antibiotic treatment
• Antibiotic use for assumed airway infection within the last 24 hours before admission to hospital
• Time from initiation of antibiotic therapy to screening >48 hours