Health experts say the surge in coronavirus cases in Europe should serve as a warning to the U.S. not to drop its safeguards too early. Optimism is spreading in the U.S. as virus deaths are plummeting and states are easing restrictions. But across Europe tighter restrictions are returning amid a surge of cases that are straining some hospitals. The pandemic’s diverging paths on the two continents can be linked to the more successful vaccine rollout in the U.S. and a wave of more transmissible variants in Europe. Health experts add that some areas of Europe also were too quick to relax distancing requirements.