Japan’s COVID-19 case numbers have plummeted to the lowest in nearly a year just as other parts of Asia are struggling with surging infections, leaving health experts perplexed and raising concerns of a winter rebound.

New daily cases in Tokyo dropped to 87 on Monday, the lowest tally since Nov. 2 last year, and a precipitous decline from more than 5,000 a day in an August wave that hammered the capital’s medical infrastructure.

The pattern is the same across the country.

After a slow start, Japan has made rapid progress in its vaccination campaign and almost six months of emergency distancing restrictions have likely helped stem the spread of the virus.

Nevertheless, the speed with which a wave of infections and hospitalizations fueled by the infectious delta variant has ebbed away has confounded the experts.

Kyoto University’s Hiroshi Nishiura is among those who believe the summer spike in cases and subsequent plunge were mainly due to trends in human activity.

Infectivity, as measured by the effective reproduction number, is correlated with holiday breaks, he said.

“During the holidays, we meet persons whom we seldom meet up with, and moreover, there is a substantial chance to eat together in a face-to-face environment,” Nishiura, a top infectious disease modeler advising the government, said.

Recent record cases in South Korea and Singapore may be connected to some midyear holidays, and a convergence of Asian and Western holidays at the end of the year could lead to a “nightmare,” he said.

But other experts say infection trends have less to do with travel and more to do with regular, seasonal trends.