Influenza lower respiratory tract infection (LRTI) is a major cause of morbidity and mortality worldwide. While viral replication can be suppressed by antiviral treatment, excessive inflammatory responses, which are increasingly recognized to contribute to severe influenza complications, remain unopposed. Macrolides have been used widely to treat community-acquired pneumonia, and shown to exert anti-inflammatory effects in other respiratory diseases, providing clinical benefits. In this randomized, open-label, multicenter study, we aim to investigate the anti-inflammatory effects of macrolide treatment in influenza LRTI. Its impacts on the cytokine response, viral clearance, symptoms and disease resolution will be studied. Such results may lead to the development of new therapeutic approaches against severe influenza infection, and provide better insights into disease pathogenesis.

Objectives: (1) We aim to investigate the anti-inflammatory effects of macrolide treatment in patients with influenza LRTI and the impact on viral clearance. (2) To investigate the impact of macrolide treatment on influenza symptom and disease resolution.

Design: A randomized, open-label, multicenter study.

Settings: Acute medical facilities in 3 general public hospitals in Hong Kong.

Subjects, Sampling and Intervention: Adult (>/=18 years) patients hospitalized for virologically confirmed influenza LRTI who fulfill the inclusion/exclusion criteria will be randomized to receive (1) azithromycin 500 mg p.o. for 5 days (in addition to antiviral, as part of usual care), or (2) no azithromycin (ie, antiviral alone, part of usual care) after informed consent. Serial blood samples will be collected for cytokine/inflammatory response assays. Serial nasopharyngeal swabs will be collected to assess for viral clearance. Symptoms, clinical progress, radiography and adverse events will be monitored.

Inclusion Criteria:

1. established influenza A or B infection by virologic tests
2. age >/=18 years
3. present within 4 days from illness onset
4. clinical evidence of LRTI and require hospital care
5. require antiviral (oseltamivir) treatment
6. able to provide written, informed consent.

Exclusion Criteria:

1. patients on immunosuppressants
2. pregnant or lactating woman
3. known hepatic failure, end-stage renal failure, cardiac arrhythmia (e.g. prolonged corrected QT interval >450 msec)
4. known contraindications to azithromycin (e.g. allergic reaction)
5. lack of consent for study.