Japan on Sunday confirmed that a woman who tested negative and disembarked from the coronavirus-stricken Diamond Princess cruise ship later tested positive, raising questions about the effectiveness of the quarantine measures.
Health Minister Katsunobu Kato has also apologised after 23 passengers were allowed to leave without being properly tested.
The woman in her 60s returned to her home in Tochigi prefecture north of Tokyo by train after she disembarked on Wednesday, but she had a fever on Friday and tested positive on Saturday, a local official told AFP.
She was the first passenger from the ship to have tested positive in Japan after having been cleared to disembark.
The news comes as infectious disease experts and local officials have questioned the effectiveness of the quarantine period on the vessel.
"There has been a judgement that those who disembarked after testing negative had no problem, but it has now become clear that those people can turn positive," Tochigi governor Tomokazu Fukuda told a press conference late Saturday.
"We call on the government to take additional measures," he said.
Japanese authorities have said they made a decision to disembark passengers who tested negative during the quarantine period as they took measures to prevent spread of the virus — passengers were confined to their cabins, except for brief outings on open deck when they were required to wear gloves and masks and keep their distance from fellow passengers.
However, infectious diseases specialist Kentaro Iwata has said the situation on the ship was "completely chaotic" and violated quarantine procedures, in blunt criticism rarely seen in Japanese officialdom.
The Kobe University professor later said he had heard from a colleague on board that quarantine procedures had improved, but still recommended that all those disembarking the ship should be monitored for at least 14 days and should avoid contact with others.
Since Wednesday, about 970 passengers — who tested negative after the government put the ship under quarantine on February 5 — have disembarked, according to local media.
On Saturday, around 100 more passengers who had reportedly been in close contact with infected people on board were allowed to get off.
They included the last group of Japanese passengers while some foreign passengers were still waiting on board for their governments to send chartered aircraft.
With the latest disembarkation, a 14-day quarantine is expected to start for more than 1,000 crew still on board.
Many of them were not placed in isolation as they were needed to keep the ship running — preparing food and delivering meals to cabins.
Critics have charged that they were inadvertently spreading the virus throughout the ship, which has seen more than 600 cases of the potentially deadly COVID-19 disease.
Kato defended Japan's on-board quarantine, telling a TV programme Saturday there was no medical facility large enough to admit more than 3,000 people at once.