In theory, governments could potentially impose very brief travel bans when a potentially dangerous new variant is detected, just long enough to do some useful research and preparation. They can then remove the ban once – as inevitably occurs – it no longer meaningfully constrains the spread of the disease. In this way, we could maximize the gains from travel bans, while minimizing the pain.

That might, perhaps, work with well-informed “benevolent despot” governments, which are immune to political pressure and always scrupulously weigh costs and benefits. But such well-informed benevolent despots are in short supply. In the real world, governments rarely have good information when a new disease or variant first emerges.

And, once instituted, travel bans and associated migration restrictions tend to remain in place for many months past the point where there is any chance they might do good. The bans instituted in response to the initial Covid crisis remained in place long after both the original virus and later variants got in and achieved “community spread” throughout Europe and North America.