Since the onset of the Covid-19 crisis in the United States, government action taken to “flatten” the curve of disease transmission has varied dramatically among states, counties, and cities. The early epicenters — New York City, Washington State, and the San Francisco Bay Area — implemented aggressive measures in mid-March, many of which remain in place. Other states and localities opted for milder restrictions, acted much later, or barely intervened at all. Many states began unwinding restrictions weeks ago, although surging case numbers are prompting some to change course. The patchwork nature of the response helps explain the current situation: Covid-19’s spread now has many different trajectories, which partly track jurisdictional boundaries. In the third week of July, for example, Covid-19 incidence was 10 times as high in some states as in others.

Many jurisdictions have responded to the unevenness of the unfolding pandemic by battening down their borders. Nearly half the states have imposed interstate travel restrictions to date. Eight have imposed restrictions on entrants from all states, 12 have imposed them only on entrants from selected high-prevalence areas, and 4 have shifted between these positions. A common feature of these orders is a requirement that entrants — both residents and nonresidents — self-quarantine for 14 days, although news reports suggest that states are not actively monitoring compliance. Several states recently modified their orders to permit proof of negative tests for Covid-19 infection in lieu of self-quarantine. A stark illustration of the shifting geographic dynamics is that some states, such as Florida and Texas, that months ago targeted “fugitives” from Covid-19 hotspots in the Northeast now find the tables turned.

Other federations have turned to controls on movement, too. Interprovincial travel restrictions have been a prominent feature of the Covid-19 response in Canada; they are beginning to give way to other arrangements, such as the “Atlantic bubble,” a regional pact that lowers travel barriers across Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland and Labrador. Four of Australia’s eight states and territories are currently restricting all domestic entrants, three restrict entrants from certain states, and one maintains an open border. In Germany, the federal and state governments have agreed to frame travel bans around small, local areas of outbreaks within states.

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