Japanese health officials and experts on a government panel acknowledged Monday that the quarantine of the virus-hit cruise ship Diamond Princess was not perfect, but defended Japan's decision to release about 1,000 passengers after 14 days.

The officials said Japanese health authorities faced tough challenges in dealing with a foreign-operated ship that required international negotiations in the absence of established rules in such a crisis.

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"The ship was not designed to be a hospital. The ship was a ship," said Shigeru Omi, a former regional director for the World Health Organization. "Of course isolation was not ideal as would be expected from a hospital, so in my view, although the isolation was somehow effective, to a large extent it was not perfect."

More than 690 people were sickened on the ship and three died.

Omi, a public health expert who heads the Japan Community Health Care Organization, said it was the best they could do. While some people criticized Japan for confining more than 3,700 passengers and crew on the ship in what they called a botched quarantine, he said it was not feasible to test and relocate all of them for quarantine elsewhere.

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Some medical experts who helped on the ship have said the quarantine was poorly managed.

On Monday, the health ministry said a quarantine official and a government employee who helped on the ship had tested positive and were hospitalized, bringing the number of confirmed infections among government officials to six.

Japanese passengers who did not share a room with patients, tested negative and had no symptoms at the end of the 14-day quarantine period were allowed to go home on public transportation. The sight of them traveling on bullet trains and buses with other people was viewed as an alarming sign of a Japanese lack of a sense of crisis.

Omi, however, said the passengers who passed the criteria should be treated like anyone else, and those who frequent public places in the community should be deemed equally at risk. He said the ship was a condensed version of what is happening in Japanese communities.

The number of patients in the country continues to rise, and Japan is now at a critical juncture, experts say. It has about 160 cases outside the ship, including a dozen new cases reported Monday.

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In their assessment of the spread of the virus in Japan, the 12-member experts' panel raised concern about the growing number of cases whose timing, location and source of transmission are unknown.

At least 18 American and several Australian former passengers have tested positive after returning home. Experts said infections are also difficult to discover because COVID-19 can be transmitted during its incubation period by asymptomatic patients.

In Japan, a former passenger in her 60s who tested negative days before getting off the ship then tested positive after taking a train home. Experts said it was unfortunate but within their expectations.

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There is a "small possibility" of scattered cases that will create small clusters in communities after passengers return, said Norio Ohmagari, an infectious disease expert at the National Center for Global Health and Medicine. "So I'd like to call upon all the travelers who disembarked from the ship ... to report to the authorities as soon as possible" if they develop any symptoms, he said. "By doing that, we can prevent another cluster from happening."

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Most of about 1,000 crew members remain on board the ship under quarantine. Because of the need to run the ship and serve the passengers during the original quarantine, crew members could not be properly isolated.

