TOKYO - As the first passengers cleared of the coronavirus began disembarking the Diamond Princess cruise ship off Yokohama on Wednesday (Feb 19), experts piled in on the Japanese government's handling as a bureaucratic bungle that led to the huge surge in cases on the ship.

About 440 passengers, out of 3,000 or so on board, were allowed to leave the ship at the close of a 14-day quarantine period that began on Feb 5.

Another 79 confirmed on Wednesday brought the total number of cases to 621. This has fuelled concerns that the ship was effectively an incubator for the coronavirus. More than half of the positive cases, or 322, were asymptomatic.

Dr Kentaro Iwata, an infectious diseases expert at Kobe University who worked on the severe acute respiratory syndrome (Sars) in China and Ebola in Africa, said he was asked to leave the ship on Tuesday after he had raised concerns with the way things had been done.

He said in a widely-shared Youtube video on Tuesday night that the Diamond Princess had effectively become a "Covid-19 mill".

"The cruise ship was completely inadequate in terms of infection control. There was no distinction between 'green' zones that are free of infection and 'red' zones, which are potentially contaminated by the virus," he said.

He added that it was "completely chaotic", with some medical staff and government officials showing absolutely no regard for health and safety protocol. This meant they could have caught the virus, and spread it further to other people.

At least two government officials who worked on board the cruise liner have tested positive despite wearing personal protective equipment, and Dr Iwata has quarantined himself for two weeks.

Health Minister Katsunobu Kato told the Diet, Japan's Parliament , in response to the video: "We address issues on the same day if they are raised by specialist physicians who are members of the infection prevention team after they check the ship."

The Japanese authorities, on their part, also insist that most infections had occurred before the ship's quarantine period began on Feb 5, with the exception of some crew members as well as some family members who live together in a cabin.

The steady rise in cases, it reasoned, was because of the gradual testing of more passengers on board the ship.

"The decline in the number of confirmed cases, based on reported onset dates, implies that the quarantine intervention was effective in reducing transmission among passengers," the National Institute of Infectious Diseases said on Wednesday.

"Transmission towards the end of the quarantine period, which is scheduled to end for most passengers on Feb 19, appears to have occurred mostly among crew or within passenger cabins," it added.

This reassurance appeared to hold little weight with foreign governments that are arranging chartered flights home for their citizens, who will go through another 14-day quarantine.

While the United States Centres for Disease Control and Prevention had said on Feb 8 that "remaining in your room on the ship is the safest option to minimise your risk of infection", it now cites the "dynamic nature of the outbreak" as the reason the Americans on board should be on a chartered flight home.

An elderly couple on the ship who live in Hiroshima told the Tokyo Shimbun newspaper that they had been tested on Feb 4. Their results came back negative three days later.

But they are not being tested again before they leave the ship - as government officials say the risk is "extremely low" of their being infected after the quarantine period began.

Still, they told the newspaper that they felt uneasy about not being retested: "The government has not been able to put the situation under control and are acting too casually. We do not even know how the virus is being spread."

Dr Iwata, on his part, said in the video: "I never had fear of getting infection myself because I know how to protect myself and how to protect others and how infection control should be."

He added: "But inside the Diamond Princess I was so scared, I was so scared of getting Covid-19 because there was no way to tell where the virus is. There was no green zone, no red zone, the virus could be everywhere and everybody was not careful about it."

This led renowned Japanese neuroscientist Kenichiro Mogi to comment on Twitter: "It's a pandemic on board because we don't do what we need to do in response and data collection."

Dr Michael Ryan, the director of the World Health Organisation's (WHO) health emergencies programme, also acknowledged that there had been "much more transmission than expected" on the Diamond Princess.

"It's very easy in retrospect to make judgments on public health decisions made at a certain point," he said, adding: "The authorities in Japan are adjusting to that reality now and taking the necessary public health measures with other countries to evacuate people and deal with their follow-up in a different way."

Japan is facing a growing outbreak on land as well, registering 10 new cases on Wednesday, bringing its total tally to 84. This is nearly three times the 29 cases it had reported just last Thursday (Feb 13).

Dr Hitoshi Oshitani of the Tohoku University Graduate School of Medicine, a former expert with the WHO who was in Singapore last week to meet Ministry of Health officials, wondered if Japan had missed the boat to control the outbreak.

He told a media briefing in response to a question by The Straits Times: "Everybody has been too late in countermeasures - Japan, China, the WHO."

He said many governments had resorted to the "very basic, 19th century strategy" of containing the outbreak - which has proved ineffective in the face of a new threat in which asymptomatic patients may also spread the illness.

He added that this was likely why Japan has not managed to trace the root cause of all its cases, with at least four loose chains of transmission.

"My major concern is that in coming weeks, we may see very large outbreaks somewhere in Asia or Africa," he said. "If this does happen, we will continue to have imported cases from different sources, not just from Wuhan but from other places."