In late August, infections surged to nearly 24,000 per day and daily deaths peaked at over 65. But since then, COVID-19 infection and death rates have fallen to their lowest seen since early 2020, with Japan recording an average of 108 cases and one death per day in the last week.

Japan’s drop in infections is likely related to a number of factors, says Karen Grépin, a public health professor at Hong Kong University. For one, 77% of Japan is fully vaccinated after its campaign got off to a slow start. Japan has also maintained relatively strict border controls and adopted near universal mask wearing and intensive contact tracing measures even as it avoids mass lockdowns.

Japan isn’t alone. Evidence of COVID-19 is also fading in other Asian countries as disparate as Indonesia, India, the Philippines, Singapore, Thailand, and Cambodia. To be sure, countries like South Korea and Vietnam are in the midst of record COVID-19 surges, but the region-wide trend is clear: COVID-19 cases have been dropping across Asia since early September.

After recording nearly 200,000 cases per day on Sept. 1, the region is recording 43,000 cases per day as of Thursday, according to the United Nations Office for the Coordination of Humanitarian Affairs, meaning the entire continent is logging roughly a third of the daily cases in the United States.

Asia’s COVID-19 decline comes as Delta-driven waves are fueling surges elsewhere. Europe, for example, is recording more daily cases now than it has since the beginning of the pandemic, with infections rising from just over 100,000 cases per day in September to nearly 400,000 today, according to Reuters.

Experts say that a recent uptick in COVID-19 vaccinations is likely the most important reason for Asia’s downward trend. But other factors like natural immunity from previous infections, Asia’s slower re-opening process compared to other parts of the world, and even seasonal weather patterns may be contributing to the falling COVID rates. Still, the factors that have eased Asia’s latest COVID wave may not be able to fend off what likely lays in store: a surge of cases from the highly-mutated Omicron variant that seems to spread faster and better evade vaccines than its predecessors.

Vaccines and borders

Asia’s vaccination campaigns got off to a slow start compared to those in U.S. and Europe, but many Asian countries quickly caught up.

Singapore is now one of the world’s most inoculated countries with 88% of its population fully vaccinated, and caseloads there are dwindling. The city-state is recording roughly 750 infections per day after reaching a record of more than 3,500 cases per day in October. Malaysia and Cambodia have fully vaccinated 78% and 81% of their populations, respectively, and COVID-19 cases have plummeted in those countries too.

High vaccination rates are a “huge factor” in declining cases in these Asian countries, says Ashley St. John, an immunologist at the Duke-NUS Medical School in Singapore. “Vaccination reduces the spread of infection and also makes it more likely that the infections do still occur are mild and often sub-clinical.”

Like Japan, many countries in Asia have also been cautious in re-opening efforts, which may be keeping case counts low. Singapore experienced a surge in COVID-19 infections amid plans to ‘live with the virus’ and unlock its borders this fall. But even as re-opens, Singapore is restricting group gatherings and how many people can dine together at restaurants. Malaysia, meanwhile, has only partially re-opened its borders, allowing just a limited number of tourists to visit the tourist island Langkawi without quarantine.

“In Europe and North America, it was really common to see governments say, ‘We’re done with COVID measures, we’re done implementing all these mandates,'” says Grépin. “In Asia, countries have been much more risk averse in terms of the ways in which they’ve been opening up.”

Natural immunity and the weather

But vaccination rates and social distancing measures may not fully explain Asia’s case decline.

Indonesia, the Philippines, and India, have fully vaccinated 37%, 36%, and 35% of their populations, respectively, well below levels that might be required to achieve vaccine-induced herd immunity. Still, infections are dropping in all three countries.

Indonesia is recording its lowest level of daily COVID-19 infections since April 2020, with cases falling to 236 from 50,000 in July. The Philippines and India are also recording their lowest daily COVID rates in 18 months.

Michael Toole, an epidemiologist at Australia’s Burnet Institute, says improving vaccination rates may be helping. But these countries have likely achieved a “high level of natural immunity,” with Delta-driven waves exposing large portions of their populations to the virus during massive surges earlier this year, he says.

Another potential explanation, at least in some areas, is the weather. While much of Europe may be moving indoors as temperatures drop, people in Asia are transitioning to outdoor activities after brutally hot summers.

“There is also something to be said for seasonality as more pleasant temperatures lead to less crowding,” says Jason Tetro, a microbiologist based in Canada and author of The Germ Code. “In places like Japan, Singapore, and Indonesia, it could also be in those very hot summer months when people simply need to get into the cooler indoor air.”

But ultimately, scientists say that much of the why and when of COVID-19 waves remains a mystery.

“Although we are two years into COVID-19, we still have few answers regarding surges and drop-offs,” says Tetro.

Omicron

The new Omicron variant may be poised to disrupt Asia’s declining COVID rates. Emerging research shows that its many mutations may make the virus more likely to bypass vaccine protection and re-infect people who previously contracted COVID-19.

It is too early to know if Omicron will take hold in other countries like it has in South Africa, where the strain is driving a surge of new infections. But early signs suggest that it might be more transmissible, if less severe, than variants like Delta, meaning it could become dominant globally.

“If Omicron is more transmissible, it will out compete [Delta],” says Toole.

Grépin says that governments like Japan and Singapore are right to delay re-opening plans as the threat of Omicron looms. Such caution will allow them to “pause long enough to better understand, and reinforce domestic measures to prepare for a scenario that could lead to many, many more cases,” she says.

But ultimately, continuing to improve vaccine rates remains the best protection against the new variant.

“All evidence to date suggests that vaccines protect from severe disease caused by variants,” says St. John. “The risk of new waves will be closely linked to the level of vaccine coverage in each country.”