RecruitingPhase 1Drug trialThymalfasin (Thymosin Alpha 1; Ta1) as an Enhancer of Vaccine Response Among Older Adults Receiving Booster Doses of COVID-19 VaccineThe 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.