Aboard the Diamond Princess cruise ship, “substantial transmission” of COVID-19 started prior to Feb. 5, when the health ministry quarantined the vessel and asked passengers to stay in their rooms, according to a state-run medical institution that analyzed data related to hundreds of patients.

A preliminary conclusion by the National Institute of Infectious Diseases (NIID) suggests that secondary transmission among passengers during the 14-day quarantine may not have been as rampant as initially feared by some experts.

However, transmission toward the end of the official quarantine period “appears to have occurred mostly among crew or within passenger cabins,” the institute added in its report published Wednesday.

This means that isolation among crew members and passengers sharing rooms may not have been enough to prevent secondary infection, the report said.

“It should be noted that due to the nature of the ship, individual isolation of all those aboard was not possible. Sharing of cabins was necessary, and some crew had to continue to perform essential duties for the functioning of the vessel with passengers aboard,” the institute said.

The findings are based on NIID’s analysis of 184 confirmed patients whose start date for symptoms could be confirmed.

Of the 184 cases, 33 began showing symptoms before Feb. 5, and another 89 started from Feb. 6 to 9. Given the probable incubation period of two to 14 days, most people are believed to have become infected before the start of the quarantine, the report says.

Experts said the data in the report points to two infection peaks: one triggered by exchanges among passengers — such as during parties and buffet meals before Feb. 5 — and the other caused by service delivery by crew members after the quarantine began.

The 184 patients, however, represent only a small portion of the about 3,700 passengers and crew members aboard the ship when the quarantine started.

By Tuesday, 2,404 passengers and crew members had undergone COVID-19 tests, and as many as 542 — or more than 1 in 5 people aboard — had tested positive.

Still, the institute said the quarantine was effective in reducing transmission within 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 institute argued.

A passenger’s luggage is loaded into a vehicle after he disembarked from the coronavirus-hit Diamond Princess cruise ship in Yokohama on Thursday. | REUTERS

The report followed criticism from Kentaro Iwata, a professor at the infectious disease division of Kobe University Hospital. Late Tuesday, he posted a video clip to YouTube strongly criticizing the “chaotic” situation on the Diamond Princess, claiming there were no clear distinctions between safe and clean “green zones” and potentially dangerous “red zones.”

But on Thursday, Iwata apologized and deleted the video. During a news conference hosted by the Foreign Correspondents’ Club of Japan the same day in Tokyo, Iwata said he removed the clip because he was informed of significant improvement in zoning systems within the ship and because the NIID report suggested that the risk for passengers was significantly lower than initially expected.

The removal of the video came after Yoshihiro Takayama, an acquaintance of Iwata and a doctor working on the Diamond Princess, pointed out what he said were errors in Iwata’s description of the situation. The message was posted on Takayama’s Facebook page and later went viral.

During Thursday’s news conference, Iwata said data in the NIID report suggests that most passengers didn’t suffer secondary infections on the ship after Feb. 5 but that some crew members may have.

Iwata insisted that he still believes what he said in his original video, including his description of the “chaotic” situation on the ship.

In an interview with The Japan Times, professor Shigeru Sakurai of Iwate Medical University criticized Iwata, saying the infectious disease expert team sent to the ship did divide the vessel into several zones in an effort to properly manage the risk of infections.

Some crew members traveled between the sections, but the expert team instructed the staff about steps they could take to prevent infection, he said.

RELATED PHOTOS A passenger disembarks the Diamond Princess cruise ship in Yokohama on Thursday. | AFP-JIJI

A passenger’s luggage is loaded into a vehicle after he disembarked from the coronavirus-hit Diamond Princess cruise ship in Yokohama on Thursday. | REUTERS