Before the U.S. Navy hospital ship Comfort steamed up the East Coast from Virginia and docked at Manhattan’s Pier 90 to help the embattled city’s overwhelmed health care system, the floating hospital and its sister vessel, the U.S.N.S. Mercy, had for years operated quietly on the peripheries of American naval power, supporting humanitarian efforts and offering medical care to those in need.

The Comfort arrived in Manhattan on March 30, ostensibly for a similar mission: It would treat noncoronavirus medical cases in order to ease the flow of patients in the city’s civilian hospitals. But as the pandemic’s contours changed — and area hospitals questioned the Trump administration on why the ship had taken on so few patients — the Comfort’s mission has shifted and the ship will now allocate 500 of its 1,000 beds for severe coronavirus cases, Navy officials said on Tuesday.

The announcement that the Comfort will take some of New York’s worst coronavirus cases represents a stark about-face for the Navy, which warned as recently as Friday that the Comfort was “not configured to provide treatment for infectious diseases.” The Mercy, which was sent to Los Angeles, has not been asked to take on coronavirus patients, according to Navy officials.

But keeping the coronavirus off the Comfort has already proved challenging. One unidentified crew member tested positive on Monday and was being kept in isolation, Navy officials said. An additional five patients on board for other ailments later tested positive, but officials said the crew member had not interacted with any patient. On Tuesday, Vice Adm. Andrew Lewis, the flag officer overseeing the Comfort’s efforts, said that getting the ship ready for coronavirus patients required “minor configurations on the ship.”