In mid-August, not long after the Tokyo Olympics had wrapped up, the situation in Japan looked grim to Dr. Hideaki Oka, an infectious disease expert at the Saitama Medical University Hospital outside Tokyo.

As he treated COVID patients at his hospital, Japan was in the grip of a fifth wave of infections. New cases nationwide had surged to around 25,000 a day, and the country's medical system was being stretched to its limits.

By late September, cases had plunged, and Oka is now getting a respite, of sorts.

"We have had zero COVID patients in our hospital for two months straight," he explains, "so we've been able to concentrate on general medicine just as in pre-COVID times."

The turnaround has been so dramatic it has left experts and observers struggling to explain it. Cases dropped by more than 99% from their peak. Japan has seen less than one death a day in recent weeks, their lowest level since July 2020.

After the previous four waves of infections, almost as soon as local governments lifted their states of emergencies, cases quickly began to rebound. But after the most recent state of emergency was lifted on Sept. 28, there was no sudden rebound — or at least not yet.

"We've had below 50 new cases on average for eight weeks," Dr. Norio Ohmagari, of Japan's National Center for Global Health and Medicine, told a meeting on Dec. 9. about the situation in Tokyo.

His explanation: "We think it is due to countermeasures taken by many citizens and institutions, and accelerated vaccination, thanks to medical staffs' efforts and citizens' understanding."

Dr. Oka says that the Olympics provided the political motivation for the administration of then-Prime Minister Yoshihide Suga to speed up its vaccine rollout ahead of the games.

Japan had lagged behind other developed nations earlier this year, but now nearly 80% of its population of 125 million people is fully vaccinated. It just started administering booster shots this month.

The vaccine is one barrier keeping the virus at bay. Oka says that it's a social norm in Japan to keep another kind of barrier over people's mouths and noses.

"Even though I keep having no COVID patients, I haven't seen anybody on the street in Japan not wearing masks," observers Oka. "Even though they think it may not be necessary, there is pressure for people to do the same as everyone else."

Then again, Japan's neighbor South Korea has fully vaccinated more than 90% of its adult population. And masking up in public is the norm in that country too.

But South Korea is going through its worst wave of infections of the pandemic so far. After a month and a half of gradual easing of restrictions, it has had to reimpose limits on gatherings and business, starting on Dec. 3, with additional restrictions added on the Dec. 16

As was the case in the early months of the pandemic last year, Japan is trying to figure out why its COVID case numbers and fatalities remain low by international standards.

Few in Japan credit the government's response to the coronavirus. Despite accelerating the vaccine rollout, Prime Minister Suga was generally seen as being indecisive and too concerned about the economic impact of social distancing. He quit in September after less than a year in office.

And as was the case last year, media and scientific journals continue to publish hypotheses about an "x-factor," some characteristic of Japan's people or its environment that has helped it to escape more serious COVID-19 damage.

"One hypothesis is that there is something intrinsically different about the immune cells that the Japanese people might carry that is able to fight off the infection," says Akiko Iwasaki, an immunologist at Yale University, who, in a paper last year, tackled the question of "Why does Japan have so few cases of COVID-19."

In it, she mentions another hypothesis, which says that a milder variant of the coronavirus was already circulating in Japan, giving some people immunity to SARS CoV-2.

Yet another theory, she adds, holds that the delta variant in Japan has mutated so that it's lost the ability to replicate, pushing itself towards extinction.

Interesting as these hypotheses may be, Iwasaki says, "currently these are not proven theories."

"Even if the theories are right," agrees Dr. Oka, "it is not possible for Japan to keep the country closed and blocked off to outsiders. This will not last forever. The reality is another wave will definitely come."

To keep out the omicron variant, Japan closed its borders to non-resident foreigners last month, after briefly easing such restrictions, and case numbers appear to be creeping back up.

Another wave of infections, Oka adds, could quickly lay to rest any speculation that Japan is somehow exceptional when it comes to COVID-19.

Chie Kobayashi contributed to this report in Tokyo.

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