Photo/Illutration People enjoy drinking beer at outside tables at a restaurant in Osaka on Oct. 1, when the state of emergency was lifted for Osaka Prefecture. (Asahi Shimbun file photo)

COVID-19 cases could drop sharply without taking strong measures to stem the flow of people, according to researchers trying to explain why infections have rapidly declined in Japan. 

The researchers said the results of their study show that the government should be more cautious than ever about imposing lockdowns or taking measures to curb the movement of people.

The team, including Taisuke Nakata, an associate professor at the University of Tokyo’s Faculty of Economics, tested hypotheses to unravel the mystery behind a sharp decline in COVID-19 cases during the fifth wave of infections, which hit Japan this summer.

They presented the results of their study at a meeting of the health ministry’s advisory panel on the pandemic on Nov. 9.

In Tokyo, the number of new COVID-19 cases began surging from late July and started plunging from late August.

Rapid progress in Japan’s vaccine rollout is attributed to greatly helping reduce new infections.

But the research team decided to investigate other factors behind the sudden drop in COVID-19 cases, saying the inoculation program alone does not fully explain the plunge in infections and the timing.

The team first created a scenario where COVID-19 cases had continued rising since mid-August as the movement of people stopped declining and the vaccination rate rose.

The researchers then tested various hypotheses to determine what factors helped close the gap between the reality, where infections sharply dropped, and the hypothetical scenario.

They calculated the number of new COVID-19 cases, assuming that the Delta variant was under 1.5 times less transmissible than the Alpha variant, which raged across the country during the fourth wave of infections this spring.

The results showed infection numbers were close to the level of the actual figures.

The team also crunched the numbers on the assumption that an outbreak of the novel coronavirus occurred on a 120-day cycle. They found the infection curve again resembled the one actually seen during the fifth wave.

The researchers said there are various possible reasons behind the 120-day cycle of outbreaks. They include mere coincidence and changes in people’s behavior according to fluctuations in infection numbers.

The team tested another hypothesis that people had avoided settings that pose a high infection risk out of fear that no hospitals could admit them even if they caught the virus and became seriously ill, due to a severe strain on the health care system.

To examine the theory, the researchers based their calculations on data obtained from tweets about the extent that people refrained from going to karaoke parlors and drinking parties.

They found the infection curve was similar to the actual one, but they noted that this hypothesis does not explain the continued decline in infections from October onward.

The team concluded that COVID-19 cases could drop sharply without taking steps to further restrict the flow and movement of people.

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