But the reversal is not just a story of the European Union and its member countries working out the early kinks, and in fact their vaccination campaigns remain far from trouble-free. Major political differences between the United States and Europe set them on divergent paths.

Europe has plenty of people who distrust the shots and their governments, but vaccine resistance in the United States is more widespread and vehement, particularly among conservatives, and falls more sharply along partisan lines. The E.U. vaccination effort has slowed recently, but not like the U.S. drive, which has declined more than 80 percent.

Policy-making in most of Europe is far more centralized than in the United States, where a jumble of federal, state and local measures yield wildly different approaches from place to place. Central governments have more control over health care and, crucially, some have been more willing to use mandates and high-pressure tactics to get people to take the shots.