TOKYO -- Three people evacuated from the Diamond Princess cruise ship were found Friday to be infected with the coronavirus, raising questions about Japan's decision to let disembarked passengers go home in a stark contrast to other nations enforcing two-week quarantines.
Nearly all passengers whose tests came back clean had disembarked from the ship by Friday, under the assumption that they could return home without posing a threat. But two Australians and an Israeli were found to have the virus when they were rechecked after arriving in their home countries, making clear that the risk of spreading the disease cannot be entirely ruled out.
Japan said at first that there would be no need to restrict passengers' activities after they disembark. But on Thursday, it began distributing cards to passengers urging them to keep an eye out for symptoms for another two weeks and avoid going out unnecessarily.
Other countries have taken a tougher approach with their returning citizens. The U.S. has placed passengers under a 14-day quarantine at military bases in Texas and California. Canada, Australia, South Korea and Israel are also keeping evacuees at military or medical facilities for two weeks.
COVID-19 tests typically look at samples collected from throat swabs, even though the virus is known for causing pneumonia as it replicates deeper in the lungs. It may not be detected in cases where levels of the virus in the patient's throat are low, leading to false negative results.
This lack of certainty makes it difficult to get a clear picture of the extent of the outbreak, an infectious disease specialist said.
Passengers should be kept under observation for another 14 days after leaving the ship, argued Kazutoshi Nakashima, a professor at Daito Bunka University in Tokyo, citing the risk of people developing symptoms even after initially testing negative.
Japan's health ministry looks to collect information on passengers who tested positive for the virus after leaving the ship, including when they were examined and how the disease was transmitted, to figure out why it initially slipped under the radar.
Some experts suggest that the lack of symptoms among passengers later found to be infected may owe to the incubation period for the coronavirus being longer in some cases than previously believed.
The Diamond Princess was placed under quarantine for two weeks from Feb. 5 based on what was understood at the time about the incubation period of the virus -- the time between initial infection and symptoms starting to appear.
But Mitsuyoshi Urashima, a professor of molecular epidemiology at Jikei University School of Medicine in Tokyo, said the virus may stay asymptomatic for longer than that "in as many as 2% of patients," citing a paper by a Chinese research group.
This suggests that some of the cases that came to light Friday may have been infected before the quarantine.
"It's to be expected that there will be people who test positive for the virus after the quarantine period," Urashima said. More infections among disembarked passengers are likely to fuel criticism against the government, which is already under fire for allowing a major outbreak outside China.
Meanwhile, the roughly 1,000 crew members stationed on the Diamond Princess are being kept on board for another two weeks, according to government sources.
A report released Wednesday by Japan's National Institute of Infectious Diseases found that the quarantine was not maintained as strictly with the crew as with the passengers, noting that crew members had to share cabins and continue performing duties.