Good morning. Travel restrictions have been one of the most effective pandemic responses — if they’re strict.

‘Viruses don’t care what passport you carry’

One of the biggest lessons of the pandemic has been the success of travel restrictions at reducing its spread. And this is a moment when they have the potential to be particularly effective in the U.S., given the emergence of even more dangerous coronavirus variants in other countries.

President Biden seems to realize this, and has reinstated some travel restrictions that President Donald Trump lifted just before leaving office.

It’s not yet clear whether Biden will impose the kind of strict rules that have worked best elsewhere. So far, he has chosen a middle ground between Trump’s approach and the approaches with the best global track record.

Many of the places that have contained the virus have relied on travel restrictions. The list includes Australia, Ghana, New Zealand, South Korea, Taiwan, Vietnam and Canada’s four Atlantic provinces. At key points, they imposed severe restrictions on who could enter.

There is a crucial word in that sentence: severe. Travel bans work only when countries don’t allow a lot of exceptions.

Barring citizens of other countries while freely allowing your own citizens to return, for example, is ineffectual. “Viruses don’t care what passport you carry,” my colleague Donald G. McNeil Jr., who’s been covering infectious diseases since the 1990s, told me.

Voluntary quarantines generally don’t work either, since many people don’t adhere to them. Some take mild precautions and still describe themselves as “quarantining.” As Donald says: “For it to work, it has to be mandatory — and actually enforced. And not at home.”

Australia versus the U.S.

Australia crushed the spread of the virus in the spring partly by ending its voluntary quarantine and requiring all arrivals, including Australian citizens, to spend two weeks in a hotel. The military then helped enforce the rules. China and some other Asian countries took similar steps. In eastern Canada, tough entry rules were “one of the most successful things we’ve done,” Dr. Susan Kirkland, a Nova Scotia official, has said.

Travel bans had such a big effect, Dr. Jared Baeten, a prominent epidemiologist, told me last year, that public-health experts should re-examine their longtime skepticism of them. “Travel,” he said, “is the hallmark of the spread of this virus around the world.”

Last year, the U.S. became a case study in the ineffectiveness of limited travel rules after Trump announced a ban on entry from China. Because it didn’t apply to U.S. citizens or their immediate family members, among others, and because Trump did little to restrict entry from Europe, the measures had little effect.

The Biden administration now risks a repeat.

Infectious variants of the virus that are spreading in Brazil and South Africa could be even more dangerous than a strong new variant found in Britain, scientists say. In response, Biden is restricting entry from Europe, Brazil and South Africa, but the policy has multiple exceptions: Americans can return home from these places if they have recently tested negative, even though the test result may not be current.

The politics of travel bans are certainly thorny. Businesses worry about the economic impact (as The New Yorker’s Lawrence Wright noted in a fascinating radio interview with Terry Gross). Progressives worry about stoking anti-immigration views. And it’s already too late to keep the variants out of the U.S. entirely.

Yet travel restrictions can still save lives. The U.S. is in a race to vaccinate as many people as possible before they contract the virus, and the new variants are the biggest new challenge in doing so. “I am worried about these variants,” Dr. Vivek Murthy, the co-chair of Biden’s virus task force, said on the first episode of Ezra Klein’s Times podcast.

The U.S. travel restrictions will almost certainly have some impact by keeping out some infected people. But Biden’s policy stops short of minimizing the virus’s spread.

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• The drug maker Merck abandoned two vaccines because neither produced a strong immune response. The U.S. has authorized two vaccines and will probably need more to subdue the virus this year. Here’s how quickly countries are vaccinating their populations.
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