MIAMI — Being trapped aboard a ship with thousands of people as the new coronavirus spreads around the world isn’t the kind of vacation envisioned by most travelers.

Cruise lines are struggling as they try to reassure nervous passengers at the same time that some of their ships are being turned away from ports by officials worried about the virus’ spread.

They also are moving Asia-based ships to other destinations such as Australia and Alaska, and are turning away potential travelers who have recently been in certain regions of Asia and Europe.

Uncertainty over the extent of the outbreak caused stocks of the largest cruise line companies to plummet over the past week. But the fear thus far seems to be psychological: While travel advisers have noticed fewer last-minute bookings for the normally popular time that extends from January through March, they said they weren’t seeing many cancellations yet.

“The cruise industry will take a hit. We just don’t know how big it will be yet,” said John Thomas, a professor at Florida International University.

Some travelers are spooked by reports of a virus outbreak that killed 10 people and sickened 700 aboard a cruise ship quarantined for two weeks in Japan earlier this month.