Heart failure (HF) is one of the most common causes of hospital admission in Canada and costs the Canadian healthcare system over $1 billion annually. Influenza vaccination is an inexpensive strategy to prevent influenza infections and reduce an important trigger for HF decompensation and hospital readmission. Yet, the optimal timing of vaccine administration remains unclear. When patients with HF are admitted to the hospital with an acute decompensation in advance of, or during, the 'flu season', this can be an ideal time to administer the vaccine. However, patients with acute HF decompensation have significant inflammatory injury, and may have substantially impaired immune responses; thus vaccine administration while admitted during an acute decompensated HF episode may not lead to high anti-influenza antibody titres. A more effective strategy can be to vaccinate after the decompensation has resolved, when patients are more stable. The FLU-HF randomized trial will determine whether administering the influenza vaccine to patients admitted in-hospital with an acute HF decompensation or waiting until they have stabilized as an out-patient leads to an improved anti-influenza response.

Inclusion Criteria:

• Age > 18 years
• Admitted in hospital with primary diagnosis of acute HF
• Prior diagnosis of chronic HF > 3 months prior to admission
• Not on inotropes, mechanical support, or IV diuretics for 24 hours
• Able to follow-up within the MUHC HF clinic as per schedule
• agree to receive influenza vaccination

Exclusion Criteria:

• Any person who does not meet the above criteria and/or who refuses to participate
• Already received this seasons influenza vaccination
• Known allergy to influenza vaccination or components of the influenza vaccination
• Unlikely to survive to discharge as per admitting physician
• Prior organ transplant
• Undergoing chemotherapy for active malignancy
• Currently randomized in another clinical study