As national travel bans and local restrictions aimed at curtailing the spread of Covid-19 pile up, including an order for California residents to shelter in place, it’s time to ask whether bans on interstate travel would be lawful. In particular, if a state can order its own residents to stay indoors, can it order residents of other states to stay away? Can a state close its borders?

I’m not saying this would be a good idea. But the scholar in me can’t help pondering its legality. Imagine two neighboring states, State A and State B. Suppose the coronavirus is rampant in State A but rare in State B. Can State B, in an effort to protect its citizens, tell the people of State A that they’re not welcome?

Many legal scholars would reply that it’s unconstitutional for one state to close its border with another. But in the case of declared public health emergency, would they be right?

The question may not be as crazy as it sounds. Governors around the country are taking stronger steps than the federal government, and a new Wall Street Journal/NBC News poll finds that that 75% of respondents have faith in their state governments to deal with the virus, compared with 62% who have faith in the federal government, and only half who have faith in President Donald Trump. These numbers suggest that although we tend to think of travel bans as something imposed at the national level, people might well support local officials who acted on their own.

We’ve been down this road before. During the yellow fever outbreak of the late 19th century, many local governments responded to rising public fear by sealing their borders, a practice headlines of the day called “shotgun quarantine.” Although the disease is actually spread by mosquitoes, popular belief at the time held that the villain was personal contact. Armed patrols began to appear in relatively uninfected areas, tasked with keeping out refugees from states or towns where yellow fever was endemic.

The patrollers were not vigilantes. As the legal scholar Polly J. Price has shown in an admirable paper, the armed bands had usually been deputized by state or municipal authorities for this very purpose — in short, local governments themselves were the ones sealing their borders.

In 1900, the U.S. Supreme Court had the chance to address the legality of these actions when Louisiana accused neighboring Texas of using yellow fever as a pretext to close its borders when in truth the rules were intended to protect local businesses from competition. The justices dismissed the case on other grounds, and have not touched the subject since.

Nevertheless, the precedents suggest that one state might well possess the power to seal its borders against another. For although the courts have long held that the Constitution protects the right to travel, including from one state to another, that freedom has been subject to a considerable caveat, articulated by Chief Justice Earl Warren in the Supreme Court’s 1965 decision in Zemel v. Rusk:

“But that freedom does not mean that areas ravaged by flood, fire or pestilence cannot be quarantined when it can be demonstrated that unlimited travel to the area would directly and materially interfere with the safety and welfare of the area or the Nation as a whole.”

This passage, along with similar language in other cases, is often pointed to as the authority for orders by national or local health officials requiring that a contagious person or persons be quarantined. Even the American Civil Liberties Union conceded in a 2008 report that “interstate travel may be restricted when there is a direct threat of disease.” Specifically, “in a pandemic, governments may either impose broad bans on travel or seek to prohibit travel only by individuals who are thought to pose a high risk.”

Given the current national mood, it’s easy to imagine a court applying similar logic should one state impose a “broad ban” excluding the residents of another on public health grounds. Consider the way many localities have had to be dragged kicking and screaming to close their public schools — a move long understood by researchers to be perhaps the single most effective tool for slowing the spread of contagion. If State A swiftly closes its schools while State B dithers, it’s easy to see why residents of State A might be uneasy around residents of State B.

Now, don’t get me wrong. I’m not advocating such measures. I don’t believe closing state borders would be either useful or wise. Experts have long been skeptical that travel bans do much good. Computer simulations teach us (as should common sense) that any restriction on movement within a country will be effective only if done early — and even then, the effect will be moderate at best. The models also show that in predicting the spread of a pandemic, the choice of a particular strategy is less important than acting swiftly. In other words, whatever a government decides to do, it ought to do fast.

So although I think the authority of a state to seal its borders might well be upheld by the courts, I also think it’s a bad idea that would in any case be arriving far too late. Still, no elected official likes to sit around seeming to do nothing. Governors are running out of new emergency measures to announce. I hope none of them tries this one. But given the nation’s current mood, one never knows.

Copyright 2020 Bloomberg.