One of my great concerns about the pandemic was that it would hinder the global mobility of people and labor, perhaps permanently. Unfortunately, my worst fears are being realized: As COVID-19 mutates, it is affecting not only tourism and business travel but also migration more generally.

Consider that after the end of the Vietnam War, the U.S. took in more than 1 million Vietnamese migrants over a 20-year period. After the Soviet Union withdrew its troops in Afghanistan, the U.S. also took in many Afghan refugees, and as with the Vietnamese migrants the results were very positive.

Fast forward to the present day: The U.S. is not on track to take in many Afghan refugees at all. The political climate on immigration has turned much more negative, but there is also what I call “the COVID talking point.” If a critic of refugee resettlement wants to freeze a risk-averse but otherwise sympathetic bureaucracy, he need only ask one simple question: “But how many of them are vaccinated?”

I am struck also by the recent decisions of Croatia and Austria to place “expiration dates” on the vaccinations of visiting tourists. Until this decision, it sufficed to be vaccinated to visit either country, though there were possible other restrictions. Now, if it is 270 days since your last vaccine dose, your vaccinated status will no longer get you into the country.

This is especially discouraging because Croatia had been one of the most open countries to visitors. It also heralds a broader policy of continually shifting standards and uncertainty about travel restrictions.

In the U.S., President Joe Biden’s administration is now pushing third booster shots for people who already have been vaccinated. That might be a good idea, but it too creates additional uncertainty for travel and migration — and for social interaction more broadly. If three doses are so important, should people be allowed to travel (or for that matter interact indoors) with only two doses? The bar is raised yet again.

Of course the issues do not end with the third dose. If the efficacy of the second dose declines significantly in less than a year, might the same happen with the third dose? How long before four doses are necessary, or maybe five? What counts as being “sufficiently vaccinated”?

Many Americans seem to be keen to get their third dose, but by the nature of counting that number is fewer than the number willing to get two doses. Furthermore, many people might just tire of the stress of dealing with an ongoing stream of obligatory booster shots and stop at one or two.

The big international winner from all this is likely to be Mexico, which has remained an open country and is not relying on vaccine passports. In general I do not admire Mexico’s lackadaisical COVID-19 response, but the country may end up in a relatively favorable position, most of all when it comes to tourism and international business meetings.

As for the U.S. and Europe, the temptation to escalate required safety measures is understandable. But the previous vaccine standards were largely workable ones. If they are made tougher, they might break down altogether.

Tyler Cowen is a Bloomberg Opinion columnist. He is a professor of economics at George Mason University and writes for the blog Marginal Revolution. His books include “Big Business: A Love Letter to an American Anti-Hero.”