There are several clinical trials currently underway or recruiting at Mayo Clinic related to Covid and long Covid. Here is a listing of available trials using the search term "covid"
https://www.mayo.edu/research/clinical-trials/search-results
You can search using other terms here: https://www.mayo.edu/research/clinical-trials

You can also contact research to ask questions about eligibility and availabiliyt
Non-cancer clinical studies questions
Phone: 800-664-4542 (toll-free)
The Mayo research team can answer questions, discuss your needs and help locate research studies that match your interests. They can also provide interpreter services for non-English-speaking patients.