Thymalfasin (Thymosin Alpha 1; Ta1) as an Enhancer of Vaccine Response Among Older Adults Receiving Booster Doses of COVID-19 Vaccine

The goal of this research is to learn more about ZADAXIN® (trade name; thymalfasin generic; Ta1 for short) and determine if Ta1 has any benefit in increasing the immune response to the COVID-19 vaccine. Ta1 has been shown to stimulate the immune system to fight infections. This research study will test the safety and possible harms of Ta1 when it is given to people at different dose levels before COVID-19 vaccination.

Inclusion Criteria Subjects who meet all of the following criteria will be eligible to participate in the study: 1. Age 65 or greater. 2. Able and willing to provide informed consent or have consent provided by a legally authorized representative (LAR). 3. Scheduled for SARS-CoV-2 mRNA vaccination booster dose. 4. If a male subject, the subject must agree to use barrier contraception (ie, condoms) from Day 1 through 30 days following the last dose of study drug. Exclusion Criteria Subjects who meet any of the following criteria will be excluded from participation in the study. Laboratory related exclusion criteria should be assessed using historical records and lab results available in the subjects' electronic medical records. 1. Hypoxemia for any reason, defined as either oxygen saturation (SpO2) ≤ 93% on room air or a requirement for supplemental oxygen support. 2. Participants with one of the following: * Acute liver failure defined as INR ≥ 1.5 and altered mental status in a patient without cirrhosis or pre-existing liver disease. * Acute kidney failure defined as an increase in serum creatinine of ≥0.3 mg/dL within 48 hours or ≥50% within 7 days OR urine output of \6 hours * Heart failure with NYHA functional classification III or IV. 3. Advanced cancer being treated with cytotoxic chemotherapy. 4. Participants have end stage renal disease requiring hemodialysis or peritoneal dialysis, or chronic kidney disease with GFR \< 30 mL/min/1.73m2 5. Participants with a known history of cirrhosis and Child-Pugh score B or C. 6. Participants who are moderately or severely immunocompromised defined as: * Are receiving active treatment for solid tumor and hematologic malignancies. * Have hematologic malignancies (e.g., chronic lymphocytic lymphoma, non Hodgkin lymphoma, multiple myeloma, acute leukemia) and are known to have poor responses to COVID-19 vaccines, regardless of the treatment status for the hematologic malignancy. * Received a solid-organ or islet transplant and are receiving immunosuppressive therapy. * Received chimeric antigen receptor T cell (CAR T-cell) therapy or a hematopoietic cell transplant (HCT) and are within 2 years of transplantation or are receiving immunosuppressive therapy. * Have a moderate or severe primary immunodeficiency (e.g., severe combined immunodeficiency, DiGeorge syndrome, Wiskott-Aldrich syndrome, common variable immunodeficiency disease). * Have advanced or untreated HIV infection (defined as people with HIV and CD4 T lymphocyte cell counts \