Quarantines continue to frustrate travelers and strangle airlines a year into the pandemic, with the threat from highly infectious coronavirus variants meaning enforced isolations are mostly getting longer and stricter rather than easing up.

Even as vaccines embolden countries like Israel and the U.K. to plot paths to reopening, authorities around the world are tightening the screws to stop COVID-19 mutations from slipping through quarantine models designed to contain a less aggressive virus. With questions hanging over the efficacy of vaccines on mutated strains, this new front in the public health battle is damping hopes of a swift rebound in international air travel.

While U.K. Prime Minister Boris Johnson said Monday that foreign travel could start as soon as May 17, triggering a surge in flight bookings, England has only just put in place its toughest border curbs of the pandemic, imposing 10-day hotel quarantines for British and Irish nationals and residents arriving from dozens of countries.