Newly released data from a Japanese research institute appears to back the government's case that its quarantine strategy for the Diamond Princess cruise ship was successful in stemming contagion of COVID-19 among passengers.

Japan has been criticised for its handling of the quarantine, as more than 620 people on board have been infected with the virus and two elderly passengers have died.

The U.S. Centers for Disease Control and Prevention (CDC) has said the quarantine may not have been sufficient.

But a report released on Wednesday by the National Institute of Infectious Diseases (NIID) showed that the onset of symptoms from confirmed cases of COVID-19 peaked on Feb. 7 before tailing off to zero by Feb. 15.

Cases among crew were observed to steadily increase, peaking on Feb. 13.

“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,” according to the report. Later transmission “appears to have occurred mostly among crew or within passenger cabins.”

The NIID is Japan's top research institute of infectious diseases.

More than 620 passengers have been infected on the ship, which has been quarantined since Feb. 3, initially with about 3,700 people on board. The health ministry announced on Thursday that two passengers in their 80s had died, the first fatalities from the ship.

The NIID report was “very reassuring,” said Kentaro Iwata, an infectious disease specialist from Kobe University Hospital who had been one of the harshest critics of the quarantine. After visiting the ship, Iwata had posted YouTube videos decrying the infection controls he saw.

The videos got over a million views before Mr. Iwata took them down, saying at a press briefing on Thursday that the new data and reports of improved controls on the ship convinced him that the government had responded to his criticism.

The United States, Canada and other nations evacuated their citizens ahead of the Feb. 19 end of the quarantine. Remaining passengers have been released in batches and advised by health authorities to stay at home and monitor their condition.

The health ministry will provide assistance to those returning passengers, as there is still a risk they could develop symptoms, said Koji Wada, a professor at the International University of Health and Welfare in Tokyo.

“We can say the quarantine has been done as we planned,” said Mr. Wada, who has worked as an advisor on site at the Diamond Princess. “I'm not sure if I can say it's a success.”