TOKYO (Reuters) - As many as 91 crew of an Italian cruise ship docked in the Japanese port of Nagasaki are infected with coronavirus, officials said on Friday, as questions persist over how they might return to their home countries.

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Authorities have tested about half the vessel’s crew of 623 and are racing to screen the rest. Those who test negative will be repatriated, the government said.

The vessel, the Costa Atlantica, was taken into a shipyard in Nagasaki in February for repairs and maintenance after the pandemic prevented its scheduled repairs in China.

Earlier this year, more than 700 of its passengers and crew tested positive for the virus on the Diamond Princess cruise liner as it was docked in Yokohama.

Nagasaki authorities quarantined the Costa Atlantica on arrival in Japan, and ordered its crew not to venture beyond the quay except for hospital visits.

But prefecture officials said this week that some of the crew had departed without their knowledge, and sought detailed information on their movements.

Two other liners run by the same operator, the Costa Serena and Costa Neoromantica, with combined crews of about 1,000, are also moored in Nagasaki and due to leave by the end of April, but they have no known cases, and no virus testing is planned.

Only one of the 91 infected sailors on the Costa Atlantica has been admitted to hospital. 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 the public broadcaster NHK said the ship’s infections had helped carry the national tally of virus cases beyond 12,500 on Friday. Some 328 people have died.

The capital Tokyo confirmed 161 new cases, NHK said, below its single-day record of 201 registered a week earlier.

The Japanese Red Cross Society has begun antibody testing in cooperation with the government to check if a blood donor has previously been infected.

The Red Cross said it would use the results to assess the reliability of the test kits, but the Mainichi Shimbun daily said they would also be used to gauge the spread of the virus.

The government is considering announcing results 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.

Experts have the said the government’s slowness in rolling out antibody tests has made it hard to trace the disease and has led to infections in hospitals.

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