Vaccination Efficacy With Metformin in Older Adults

Inclusion Criteria: * Men and women 65 years and older * Willingness to provide consent and participate in all aspects of the trial including randomization to the intervention group and receiving this year's flu vaccine * Received previous year's seasonal influenza vaccine Exclusion Criteria: * History of severe renal impairment and/or eGFR ≤ 45 mL/min/1.73m2 * Excessive alcohol use (more than 14 alcoholic drinks/week) or clinical/laboratory evidence of hepatic disease (via medical history and/or AST and/or ALT ≥ 3 times upper limit of normal at screening) * Acute or chronic metabolic acidosis (via medical history and/or low serum bicarbonate (\< 22mEq/L), increased anion gap (\> 10 mEq/L)) * History of B12 deficiency within the last 10 years * Known type 2 diabetes or screening of prediabetic (HbA1c 5.7-6.4%) or diabetic (HbA1c ≥ 6.5%) * Currently taking metformin or other diabetes medications * Unwilling or unable (due to significant cognitive impairment) to provide informed consent * Terminal illness with life expectancy less than 12 months * Advanced neurological disorder (Parkinson's, ALS, MS, dementia) * Cancer or history of cancer in the past 2 years (excluding non-melanoma skin cancer) or any history of metastatic (stage 4) cancer * Significant co-morbid disease (severe chronic obstructive pulmonary disease, active rheumatologic diseases, chronic infection (HIV, tuberculosis), severe congestive heart failure (NYHA class 4), myocarditis, etc) * Myocardial infarction, stroke or hospitalization for heart failure in the last 12 months * Severe active psychiatric disorder (e.g. bipolar, schizophrenia) * Unable to complete physical performance testing due to medical conditions (at discretion of the PI) * Uncontrolled hypertension (systolic/diastolic blood pressure \>150/90 mmHg) * Unintentional weight loss \>10% in past 12 months * Allergy or history of adverse reaction to flu vaccine or any ingredients in flu vaccine, including eggs * History of Guillain-Barre syndrome following vaccination * Immunosuppressive disorders or taking immunosuppressive medications (including oral prednisone \> 10mg/day) * Current participation in another interventional study * Refusal to receive flu vaccine