With 634 infections and two deaths, the cruise ship represents the largest concentration of coronavirus cases outside China, meriting its own category in the data compiled by the World Health Organization.

The U.S. government this past week allowed 14 Americans who were infected to board evacuation flights with hundreds of passengers who weren’t. Japanese officials have since let close to 1,000 passengers who tested negative walk free, even though experts fear some of them have been exposed and could later develop symptoms. Crew members were expected to start leaving this weekend.

On Saturday, the health minister admitted that 23 passengers had been released from the ship without taking a valid recent test and had traveled by public transit after disembarking this past week.

Now that the quarantine has ended and most of the passengers have left, the concern is that they could start spreading the virus on shore.

Japanese officials said they did the best they could in a fast-moving situation, as they tried to keep the virus from spreading within the country. After confirming the first cases among those on board, the authorities said, they moved to isolate passengers to reduce transmission. The government has said the quarantine was largely effective.

The ship operator, Princess Cruises, said Japanese authorities took the lead in testing and protocols. It added that the “focus has been and remains the safety, health and well-being of our guests and crew.”

In the early hours of Feb. 2, before the ship had even docked in Yokohama, Hong Kong officials informed the Japanese health ministry about the initial infected passenger.