The Florida Keys (Monroe County)
Monroe County contains the Florida Keys. More than 99% of the county population is in the Keys and not the mainland. The mainland part is virtually uninhabited Everglades National Park. About one third of the county population is in Key West.
The Keys are an interesting case for study due to their restriction of movement into the Keys during the 2020 pandemic. The Overseas Highway is the only road into the Florida Keys. 24-hour checkpoints enforce a county order for the Keys to be closed to all tourists and leisure visitors. All tourism from cruise ships has ceased.
The degree of restriction in movement is likely greater than every other locale in the US. The Keys are likely the closest any community in the continental US will get to protective sequestration (or "reverse cordon sanitaire").