About 3,700 passengers and crew members had been quarantined on the ship, which is docked in Yokohama. More than 600 of them tested positive for the virus. Nearly 1,000 passengers who tested negative finally disembarked on Wednesday and Thursday.
What was happening in the ship
NHK spoke with some passengers, who say they’re happy the ordeal is over. A 82-year-old man said it was terrifying each time he heard the ship's PA announcing more infections.
Japan's National Institute of Infectious Diseases says the virus continued to spread even after passengers were told to stay in their cabins.
It also says there is clear evidence that there was a "substantial transmission" before the start of the quarantine, and that most infections near the end of the quarantine period appears to have occurred mostly among the crew or within cabins.
Health minister Katsunobu Kato says the government has done everything possible to prevent infections on board the cruise ship.
One Japanese expert in infectious diseases who assessed the ship told NHK on Wednesday that there was not enough done to prevent the virus spreading. Professor Kentaro Iwata of Kobe University was allowed on board as a member of D-MAT, the Disaster Medical Assistance Team.
He says experts in infectious diseases protect themselves by clearly marking hazardous areas where the virus may exist, but he couldn't tell the safe areas from the dangerous ones on the Diamond Princess.
Iwata said areas of the ship were separated according to the risk of infection, but it wasn't clear where you should put on or take off protective suits. He said some crewmembers were walking around with no restrictions.
At a news conference on Thursday, he said he had heard that the ship's measures have improved significantly since he was on board, including a clear division of areas.
How to deal with infection
Professor Shigeru Sakurai of Iwate Medical University also examined quarantine measures on the ship last week. He says keeping the passengers and crew on board was the right thing to do because there was no adequate quarantine station available on land.
But he added that some of the measures taken to prevent infections on the ship were inappropriate.
Sakurai said crewmembers were wearing high-filtration face masks, but some of them loosened their masks after about 15 minutes because it was hard to breathe. He said that they may have touched their faces while doing that, and gotten infected with the virus.
Sakurai also says crewmembers followed the ship's manual by washing their hands with water and then disinfecting with alcohol, but you cannot disinfect wet hands thoroughly.
WHO expert: 'No guidelines' on how to fight virus
A World Health Organization expert who has been advising Japanese officials says the quarantine was an unprecedented situation.
"A new virus onboard a ship with 4,000 people: there are no guidelines for that," says senior epidemiologist Matthew Griffith.
He said quarantines are always a balance between caution and feasibility, and in this instance it was not possible to isolate every person. "There just weren't enough facilities available," he says.
Griffith says the world is in uncharted territory and the unprecedented nature of the coronavirus means it will take time for experts to understand the best path forward.