More than 106 million people in the United States are fully vaccinated. Airlines are resuming overseas flights. Come summer, fully vaccinated people traveling from America will once again be welcome across Europe.

But the reality is more sobering.

Globally, more new coronavirus cases were reported in recent weeks than at any point since the onset of the pandemic. The numbers are being driven by an uncontrolled outbreak in India, but also account for troubling trends among European destinations popular with Americans, from France and Germany to Italy and Spain.

“My doomsday scenario is a mixing of vaccinated and unvaccinated populations in a setting where there is high viral load and high viral transmission,” said Dr. Sarah Fortune, the chair of the Department of Immunology and Infectious Diseases at the Harvard T.H. Chan School of Public Health.

Even if international tourists could travel safely, securely and without risking the well-being of their hosts, visitors may face yet another impediment: The destinations may lack many of their usual draws. In Paris, bars and restaurants have been closed since the end of October, as are museums.