> This is a key difference between EU and USA - the Schengen agreement allows member states to temporarily "opt out" of the free travel, reinstate border controls and impose travel restrictions (up to a full closure of borders) for various purposes, and many EU countries did just that due to Covid; while in USA, as far as I understand, the constitution greatly limits the right of states to prohibit interstate travel.
The Supreme Court has directly addressed this in the case of disease epidemics and found that, in the absence of specific Congressional action preempting such regulations, states are free under the Constitution to respond to epidemic disease (and several other kinds of crisis) by enacting and enforcing travel restrictions, even though they impact interstate commerce.
While no state has adopted border closures for containing COVID-19, several have adopted quarantine policies affecting interstate travel and adopted border checkpoints as part of the enforcement mechanism.
The Supreme Court has directly addressed this in the case of disease epidemics and found that, in the absence of specific Congressional action preempting such regulations, states are free under the Constitution to respond to epidemic disease (and several other kinds of crisis) by enacting and enforcing travel restrictions, even though they impact interstate commerce.
While no state has adopted border closures for containing COVID-19, several have adopted quarantine policies affecting interstate travel and adopted border checkpoints as part of the enforcement mechanism.