This is the key thing:
"Adults 18 years and older without health insurance and adults whose health insurance does not cover all COVID-19 vaccine costs can get updated COVID-19 vaccines at no cost through the Bridge Access Program."
(from https://www.cdc.gov/vaccines/programs/bridge/index.html)
If you have trouble at a US Walgreens or CVS, I would start with that page and quote.
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