* Italian vessel docked in Nagasaki has 91 infections
* Questions persist over their return to home nations
* Prefecture officials coordinating effort with central government
* About half the 623 crew tested, officials rush to screen rest (Adds details on testing efforts; paragraphs 12-18)
By Chang-Ran Kim
TOKYO, April 24 (Reuters) - As many as 91 crew of an Italian cruise ship docked in Japan's southwestern port of Nagasaki are infected with coronavirus, officials said on Friday, as questions persist over how and when they will return to their home countries.
Authorities have tested about half the vessel's crew of 623 and are racing to screen the rest after finding one of them infected this week, fanning worries that the illness could spread wider and eventually put a strain on medical services.
Those who test negative will be repatriated, the government said, a decision hailed by officials in Nagasaki prefecture.
"We're eager to get this done as soon as possible," one of the officials told a livestreamed news conference on Friday, adding that the procedure and timing have yet to be worked out.
The vessel, the Costa Atlantica, was taken into a shipyard in Nagasaki in late February by a unit of Mitsubishi Heavy Industries for repairs and maintenance after the pandemic scuttled plans for scheduled repairs in China.
News of this week's infections has spurred comparisons with the Diamond Princess cruise liner docked in Yokohama two months ago, as more than 700 of its passengers and crew ultimately tested positive for the virus.
Nagasaki authorities quarantined the Costa Atlantica on arrival in Japan, and ordered its crew not to venture beyond the quay unless they required hospital visits.
But prefecture officials said this week they had learned some of the crew had departed without their knowledge, and sought detailed information of their movements.
A Mitsubishi official told Reuters it was seeking information from the vessel's operator, Costa Cruises, and hoped to share its findings this week.
Two other liners run by the same operator, the Costa Serena and Costa Neoromantica, with total crew of about 1,000, are also moored in Nagasaki and due to leave by the end of April, though no virus testing is planned in the absence of known cases.
The risk of straining medical services remains, although just one of the 91 infected sailors has been hospitalised. The rest, with slight symptoms, or none, remain aboard, monitored by a doctor and four nurses, the Nagasaki official said.
Hospitals are running out of beds in some parts of Japan, where public broadcaster NHK says the ship's infections have carried the tally of virus cases to 12,472, with 328 deaths.
In the latest effort to gauge the spread of the virus, the Japanese Red Cross Society has launched an antibody test in cooperation with the government to check if a blood donor has previously been infected.
The Red Cross has said it would use the result of a survey done with donors' consent to assess reliability of the antibody test kits, but the Mainichi Shimbun daily said the survey would also be tapped to estimate the spread of the virus.
The government is considering announcing the result from the first survey batch as early as May 1, the paper added.
A government advisory panel on the virus recommended such tests on Wednesday to gauge latent infections.
The government has faced criticism over its strategy on polymerase chain reaction (PCR) tests, which experts have blamed for making it tough to trace the disease and having led to infections in hospitals.
On Thursday New York Governor Andrew Cuomo said tests of state residents showed nearly 14% had virus antibodies in a preliminary survey, suggesting that about 2.7 million may already have been infected.
(Interactive graphic tracking global spread of coronavirus: open tmsnrt.rs/3aIRuz7 in an external browser.)
Reporting by Chang-Ran Kim; Additional reporting by Chris Gallagher and Kiyoshi Takenaka; Editing by Himani Sarkar and Clarence Fernandez