Photo/Illutration Quarantine officials in protective gear ask each passenger arriving on a flight from Seoul to fill in a questionnaire at the quarantine counter of Narita International Airport on March 9. (Yoshifumi Fukuda)

A team of experts predicts that if just 10 travelers infected with the coronavirus enter Japan from abroad per day, a large-scale epidemic will occur three months later. 

Their research comes as the Japanese government eyes relaxing its tightened border controls.

Hiroshi Nishiura, a professor of theoretical epidemiology at Hokkaido University who led the research team, is asking the government to carefully prepare its policy changes, taking their modeling into account, to prevent another epidemic from occurring.

That would mean limiting the number of people who can enter the country based on various risk factors.

“If many infected people come to Japan, the measures, such as the isolation and PCR (polymerase chain reaction) tests, become ineffective,” Nishiura said. “It is necessary to control the border based on the quantitative analysis.”

Japan has banned entry from 111 countries and regions. The health ministry conducts PCR tests on Japanese returning from those areas for the coronavirus. Even if they test negative, they are urged to stay at home or other accommodations for two weeks. Travelers from other areas are also requested to self-isolate for two weeks. 

Health experts have pointed out that the major cause of the epidemic in Japan from March was coronavirus carriers who entered the country from Western countries.

The team used a mathematical model to calculate the probability of how many virus carriers entering the country per day would lead to another major epidemic.

The calculations assume all the people who enter the country will take the PCR tests and self-isolate--even if they test negative.

The simulation also assumed some travelers will spread the coronavirus in Japan after entering the country because the test is not perfectly accurate and some people will not follow the isolation rules.

Their results showed, with a 99 percent probability, that if just 10 infected people enter the country per day, a large-scale outbreak requiring the country to declare another state of emergency will occur 90 days later.

If just two infected people enter a day, the probability for a second wave drops to 58 percent. If only one infected person enters, the probability is 35 percent.

Nishiura, a member of the health ministry’s cluster task force, said that he announced this data in his personal capacity as an expert.

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