Characterizing Postoperative T and B Cell Dysfunction in Cancer Surgery Patients, Using COVID-19 as a Model Antigen

Inclusion Criteria: * Patients with a diagnosis of a primary abdominal malignancy consented to undergo major surgery with a planned length of stay of 3 or more days. * Eligible patients must have signed a consent for surgical resection of the malignancy. * Adequate hematological function defined as: platelet count ≥ 100 x 109/L, absolute neutrophil count ≥ 1 x 109/L, white blood count ≥ 2.5 x 109/L, hemoglobin count ≥ 90 g/L. * Adequate organ functioning, including: total bilirubin ≤ 1.5 x upper limit of normal (ULN); AST, ALT \< 2.5 x ULN; INR \<1.5 ; CrCl\>30mL/min. * Ability to understand and provide a signed informed consent form (ICF) approved by the Institutional Review Board (IRB/IEC/REB). * Ability to comply with protocol requirements. Exclusion Criteria: * Documented significant immunodeficiency due to underlying illness (e.g. HIV/AIDS) and/or medication (e.g. systemic corticosteroids, azathioprine, cyclosporin A). Subjects may be on physiologic doses of replacement prednisone or equivalent doses of corticosteroid (\<7.5 mg daily). * History of autoimmune disease (even if controlled with medication) such as but not restricted to, inflammatory bowel disease, systemic lupus erythematosus, ankylosing spondylitis, scleroderma, or multiple sclerosis. * Allergies or any contraindication to the use of tadalafil or any components of Cialis® or the influenza vaccine (including eggs), Agriflu®. * Serious intercurrent chronic or acute illness, or other illness considered by the investigator as an unwarranted high risk for an investigational product. * Patients who have suffered a myocardial infarction, stroke, or life-threatening arrhythmia within the last 6 months. * Patients with resting hypotension (BP \<90/50 at rest) or hypertension (BP \>170/110 at rest). * Patients with cardiac failure or coronary artery disease causing unstable angina.