Each year, influenza A infection caused great mortality and morbidity, especially among the
elderly and individuals with chronic illness. Many of these patients are 'late presenters'
who are admitted to hospital a few days after symptoms onset and have developed complications
secondary to immunodysregulation. Antiviral treatment with the neuraminidase inhibitor is of
limited usage for patients who presented to the hospital 48 hours after symptom onset. Apart
from ventilatory and extracorporeal membrane oxygenation support, treatment options for these
patients are limited. Recent animal study has demonstrated that combinations of an antiviral
agent with a COX-II inhibitor can reduce mortality in mice infected with influenza virus. The
investigators therefore propose to enrol patients with severe influenza A infection requiring
hospitalization and oxygen support on a randomized controlled trial with celecoxib.