As they begin to set sail again, cruises lines are optimistic for a turnaround.

Nothing demonstrated the horrors of the coronavirus contagion in the early stages of the pandemic like the major outbreaks onboard cruise ships, when vacation selfies abruptly turned into grim journals of endless days spent confined to cabins as the virus raged, eventually infecting thousands of people on board, and killing more than 100.

It was difficult to imagine how the ships would be able to sail safely again. Even after the vaccination rollout gained momentum in the United States in April, allowing most travel sectors to restart operations, cruise ships remained docked in ports, costing the industry billions of dollars in losses each month, report Ceylan Yeginsu and Niraj Chokshi for The New York Times.

Together, Carnival, the world’s largest cruise company, and the two other biggest cruise operators, Royal Caribbean and Norwegian Cruise Line, lost nearly $900 million each month during the pandemic, according to Moody’s, the credit rating agency.

Several epidemiologists questioned whether cruise ships, with their high capacities, close quarters and forced physical proximity, could restart during the pandemic, or whether they would be able to win back the trust of travelers.

But the opposite has proved true, said Richard D. Fain, chairman and chief executive of Royal Caribbean Cruises. “The ship environment is no longer a disadvantage, it’s an advantage because unlike anywhere else, we are able to control our environment, which eliminates the risks of a big outbreak,” he said.

After months of preparations to meet stringent health and safety guidelines set by the Centers for Disease Control and Prevention, cruise lines have started to welcome back passengers for U.S. sailings, with many itineraries fully booked throughout the summer.

Carnival said bookings for upcoming cruises soared by 45 percent during March, April and May as compared with the three previous months, while Royal Caribbean recently announced that all sailings from Florida in July and August were fully booked.

“The demand is there,” said Jaime Katz, an analyst with Morningstar.

The industry’s turnaround is far from guaranteed. The highly contagious Delta variant, which is causing a surge of cases around the world, could stymie the industry’s recovery, especially if large outbreaks occur on board. But analysts are generally optimistic about its prospects. That optimism is fueled by what may be the industry’s best asset: an unshakably loyal customer base.