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Why have new COVID cases declined so quickly in Japan, and why is caution needed?

The area around Umeda Station, with fewer people about during the coronavirus state of emergency, is seen in Osaka's Kita Ward on Sept. 16, 2021. (Mainichi/Tatsuya Fujii)

OSAKA -- New COVID-19 infections have been falling quickly in Osaka after peaking at the beginning of September during the virus's fifth wave, matching a nationwide trend.

    Shigeru Omi, Japan's top COVID-19 adviser, says there are multiple factors behind the decrease. Experts point to restrictions on people going out at night and the effectiveness of vaccines. Mutations of the virus as part of its "survival strategy" have also come into focus. But despite the trend, people are being urged not to let down their guard.

    Osaka Prefecture saw a single-day high of 3,004 coronavirus infections on Sept. 1, topping 3,000 for the first time and rivaling the 3,168 infections recorded in Tokyo the same day. But just one week later, the daily figure dropped to 2,012, and by Sept. 15, it had fallen to 1,160 people.

    "There are a number of conceivable factors, but no single stand-out reason," commented Mutsuko Fujii, head of the Osaka Prefectural Government's Department of Public Health and Medical Affairs, at a Sept. 9 meeting of the prefectural government headquarters for coronavirus countermeasures.

    Looking at the data, it appears that shifts in the number of people out and about is a related factor. According to Setsuya Kurahashi, a professor in social simulation at the University of Tsukuba, there is a correlation between infections and the nighttime flow of people in Tokyo and Osaka.

    Analysis of data from SoftBank Corp. subsidiary Agoop Corp. showed that from the time the fourth state of emergency was declared in Osaka Prefecture on Aug. 2 until Sept. 8, the number of people around Umeda Station between 9 and 10 p.m. was down by about 30% compared to the same period between March 1 and April 4, before the fourth wave of infections started spreading. Meanwhile, analysis of social media showed that since August, posts on karaoke, drinking, and barbecues were down to about one-quarter of previous levels. Kurahashi commented, "After the Olympics ended, there was a lot of reporting on the strain on the medical system. A long spell of rain during the Obon holiday also led to behavioral restraint."

    Professor Takashi Nakano of Osaka University's new Center for Infectious Disease Education and Research (CiDER), said, "When people see an infection at their workplace or at home, they likely change their behavior and avoid contact with anyone they would normally not meet."

    He added, "Up until now, infections have died down after a set period. During the first four waves of the virus, after infections peaked, they declined at about the same pace, but this time the decline has been over 10% faster." He suggested that the progression of vaccinations has decreased the number of people who can be easily infected.

    The area around JR Osaka Station is seen during the state of emergency, in the city's Kita Ward at 9:28 p.m. on Sept. 16, 2021. Few people were out at night. (Mainichi/Tatsuya Fujii)

    Yoshiaki Katsuda of the Kansai University of Social Welfare, who has been involved in the treatment of coronavirus patients, pointed to weather as a factor. Since the latter half of August, there were many abnormally cool days for the season. "When the air conditioning is on, people are more concerned about their surroundings, and so it's difficult to open the window, but when it gets cooler, there's less resistance to this. I think ventilation has had some effect."

    He continued that news of virus clusters stemming from basement food floors in department stores and other such reports likely caused people to reflect on their own activities and refrain from going out. "The effectiveness of the state of emergency declaration, which was described as being in a rut, was not zero," he said.

    Some, however, have pointed out that the flow of people during the day has not been brought under control. So what other reasons are there? CiDER director Yoshiharu Matsuura, who for many years has been involved in vaccine development, has turned an eye to the virus itself.

    Viruses can only multiply in living cells. If the virus is so pathogenic that the cells die, then the virus itself can't multiply efficiently, so it continues to mutate to survive, changing its transmissibility and pathogenicity. During this process, infections increase and decrease.

    Matsuura noted that the coronavirus is more susceptible to mutation than the influenza virus, which is seasonal and undergoes only small changes before spreading in the winter. The coronavirus "hasn't been spreading among humans that long, so people and the virus are probably in the process of searching for points of compromise."

    He said it was possible that the sudden drop in infections could be considered part of the process of the repeated settling down and spread of the virus, and cautioned, "Even if infections drop temporarily, a new wave will come."

    (Japanese original by Koki Matsumoto and Satoshi Kondo, Osaka Science and Environment News Department; and Satoshi Takano, Osaka City News Department)

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