Lessons From Europe’s Covid Surge: Control Is Fragile and Losing It Is Easy

Europe seemed to have Covid-19 under control this summer, enough so that authorities let residents travel more freely among the region’s countries again.

Then tourists flew home from vacations with the pathogen, seeding fresh outbreaks.

As Americans enter a period of peak travel among their states starting with Thanksgiving, new discoveries about how Europe’s second coronavirus wave spread provide an object lesson.