Almost overnight, Japan has become a stunning and somewhat mysterious coronavirus success story.
Daily new COVID-19 cases have plummeted from a mid-August peak of nearly 6000 in Tokyo, with caseloads in the densely populated capital now routinely below 100 - an 11-month low.
The bars are packed, the trains are crowded and the mood is celebratory, despite a general bafflement over what, exactly, is behind the sharp drop.
Japan, unlike other places in Europe and Asia, has never had anything close to a lockdown, just a series of relatively toothless states of emergency.
Some possible factors in Japan's success include a belated but remarkably rapid vaccination campaign, an emptying out of many nightlife areas as fears spread during the recent surge in cases, a widespread practice of wearing masks and bad weather in late August that kept people home.
But with vaccine efficacy gradually waning and winter approaching, experts worry that without knowing exactly why cases have dropped so drastically, Japan could face another wave like this summer, when hospitals overflowed with serious cases and deaths soared - though the numbers were lower than pre-vaccination levels.
Many credit the vaccination campaign, especially among younger people, for bringing infections down.
Nearly 70 per cent of the population is fully vaccinated.
"Rapid and intensive vaccinations in Japan among those younger than 64 might have created a temporary condition similar to herd-immunity," Dr Kazuhiro Tateda, a Toho University professor of virology, said.
Tateda noted that vaccination rates surged in July to September, just as the more infectious Delta variant was spreading fast.
He cautioned, however, that the so-called "breakthrough infections" in the US, UK and other places where inoculations began months earlier than in Japan show that vaccines alone are not perfect and efficacy gradually wears off.
Daily new infections surged just weeks ahead of the Olympics, forcing Japan to hold the Games with daily caseloads of more than 5000 in Tokyo and about 20,000 country-wide in early August.
Tokyo reported 40 cases on Sunday, below 100 for the ninth straight day and the lowest this year.
Japan as a whole reported 429 cases on Sunday for an accumulated total of about 1.71 million and 18,000 deaths since the pandemic began early last year.
So why the drop?
"It's a tough question, and we have to consider the effect of the vaccinations progress, which is extremely big," said Disease Control and Prevention Center Director Norio Ohmagari.
"At the same time, people who gather in high-risk environments, such as crowded and less-ventilated places, may have been already infected and acquired natural immunity by now."
Public health experts want a comprehensive investigation into why infections have dropped off.
An analysis of GPS data showed that people's movements in major downtown entertainment districts fell during the most recent, third state of emergency, which ended on September 30.
"I believe the decrease of people visiting entertainment districts, along with the vaccination progress, has contributed to the decline of infections," said Atsushi Nishida, the director of the Research Center for Social Science & Medicine Sciences at the Tokyo Metropolitan Institute of Medical Science.
But people headed back to entertainment districts as soon as the recent emergency ended, he said, and that may "affect the infection situation in coming weeks".
Australian Associated Press
