People walk along Harajuku's famous Takeshita Street in Tokyo's Shibuya Ward, on March 28, 2020. (Mainichi/Kimi Takeuchi)

Experts in Japan have been simulating how the spread of the novel coronavirus can be tamped down, but in areas where the national government has declared a state of emergency, people's behavior must be firmly restricted, which is a task that, realistically speaking, is extremely difficult.

Akihiro Sato, a professor of data science at Yokohama City University, analyzed the numbers of 15 prefectures, including the seven where the state of emergency was declared. Based on the number of newly infected people announced by local governments, and the proportion of people who recover after being infected and showing symptoms, Sato calculated the shift in the numbers of people who were infected. Setting behavior before the period in which newly infected people increased by a large margin at 100%, Sato calculated the target percentage at which people must refrain from direct contact with others in the following two weeks for no new infections to be detected in the long term.

The results showed that in the case of Tokyo, every individual would have to cut back on the time spent on public transportation and the people they meet by 98%. For example, if one person rides on trains and buses for a total of seven hours per week, and has direct contact with a total of 100 people through work and leisure activities, that person must cut back their time on public transport to 8.4 minutes and their contact to two people per week to prevent new infections from being detected in the long term.

Fukuoka Prefecture requires the greatest behavioral restrictions, at 99.8%. Professor Sato emphasized, "Similar to evacuating from floods and tsunami, the current infection requires behavior that avoids people."

Meanwhile, Jun Ohashi, an associate professor at the University of Tokyo who specializes in human genetics, took particular note of the behavior of those infected with the new coronavirus who have symptoms and those who do not. Based on global infection data, Ohashi postulated that one person infects, on average, 2.5 people. He then calculated that in a city of 100,000 people, when there is one person who tests positive for the virus, the number of newly infected people in a day will reach 15,700 people at its peak. However, if the person who tests positive for the virus reduces their contact frequency with others by 55% of their usual behavior, newly infected people would drop to 430 people per day.

"Unless everyone, including those who are asymptomatic and those who are not infected, suppress the frequency with which they come into contact with people, the number of people who are infected will continue to rise, possibly causing the collapse of the health care system," Ohashi said. "Until we come up with vaccines and therapeutic medications, a long-term vision is essential, and it is important to change the awareness of each and every individual.

Hiroshi Nishiura, a professor specializing in theoretical epidemiology at Hokkaido University, has also calculated that if person-to-person contact can be reduced by 80%, the number of newly infected people would decline.

(Japanese original by Ryo Watanabe and Ayumu Iwasaki, Science & Medical News Department)