A surge of influenza cases in countries across Asia has led some medical experts to warn that getting sick more easily throughout the year may be the new reality.

Health authorities in Japan declared a nationwide flu epidemic on Oct. 3 after a wave of flu cases came five weeks earlier than expected for flu season, the country’s second-earliest flu outbreak in 20 years.

 More than 4,000 people in Japan were treated for influenza between Sept. 22 and Sept. 28. The country’s Ministry of Health said that equated to an average of 1.04 patients per monitored medical institution, surpassing the threshold for an epidemic. The number of patients treated for influenza has since increased to over 6,000 people from Sept. 29 to Oct. 5, corresponding to a per-institution patient count of 1.56, more than double the per-institution patient count of 0.77 for the same period last year. Across the country, from Sept. 22 to Oct. 3, 135 schools and childcare centers closed, three times more than during the same period last year.

 Twenty-eight of Japan’s 47 prefectures reported an increase in flu cases, with Okinawa reporting the highest patient per-institution count.

But it’s not just Japan. Flu cases have risen in recent weeks in other countries across the continent, including Singapore, Thailand, and India. The latter, in particular, has been gripped by the spread of the H3N2 virus in the country’s north. The seasonal H3N2 descends from the strain that caused the 1968 Hong Kong flu pandemic, which had combined human and avian influenza viruses, Dr. Martin Beer, vice president of Germany’s Federal Institute for Animal Health, the Friedrich Loeffler Institut, told DW News. [...] 

The rise in flu cases around the continent, as well as outbreaks in Europe earlier this year, have led some medical experts to warn that influenza may be evolving to spread more quickly or earlier than typical in some regions.

“The flu season has started really early this year, but in the changing global environment this might become a more common scenario,” Yoko Tsukamoto, a professor at the Health Sciences University of Hokkaido, told the South China Morning Post about the epidemic in Japan.

Nicola Lewis, director of the Worldwide Influenza Centre at the Francis Crick Institute in London, warned last year that the next global pandemic could be caused by the flu. “I think the chances that disease X will be an influenza virus are probably greater than for any other known pathogen group that I can think of,” Lewis said. Disease X refers to a hypothetical new or unknown disease that can cause a pandemic.

In Singapore, there has been a slight uptick in the number of cases with acute respiratory infections in the last two weeks, according to data from the country’s Ministry of Health, which monitors disease trends through sentinel surveillance sites including polyclinics and General Practitioner (GP) clinics. Predominantly, those cases have been the common cold rhinoviruses and influenza.

There was also a spike in flu cases last month. Dr. Zhang Qi told Channel News Asia in September that he saw a “sustained and clear surge” of patients visiting his clinic for the flu. DoctorAnywhere, a network with both physical clinics and telehealth, also noted an increase of up to 40% in flu cases.

Influenza cases are typically higher between December and March and between May and August, according to Singapore’s Communicable Diseases Agency. But the agency told CNA there’s no evidence that infections have been more severe than usual.

Dr. Kimberly Fornace, an associate professor at the National University of Singapore’s school of public health, tells TIME that there are “clear seasonal patterns described for influenza in non-tropical areas,” with a higher incidence of flu cases typically reported in colder temperatures. At least part of that comes down to human behavior: “Influenza may transmit more easily when people are spending most of their time indoors during winter,” Fornace says.

When it comes to tropical areas, though, she says there is “less understanding of these patterns.” But, anecdotally, there could be a similar pattern when it comes to wet weather. Some doctors say when the rainfall season hits in tropical countries like Singapore, people may similarly crowd indoors. Singapore has seen frequent late morning and afternoon thunderstorms in recent weeks as inter-monsoon conditions set in.

Thailand’s Public Health Ministry also warned of a surge in influenza cases in September. Between Sept. 7 and 13, data from the country’s Department of Disease Control showed more than 30,000 new influenza cases, bringing the country’s total this year to more than 555,000 with 59 deaths. That increase came as Thailand also saw heavy rainfall amid its monsoon season, as well as extreme weather that has been exacerbated by climate change.

There has been limited research into possible links between climate change and infectious diseases. A 2024 study from researchers based in Singapore suggested that climate change, including heatwaves, higher rainfall, and tropical cyclones, can increase the transmission of some viral diseases, particularly vector-borne and food or water-borne diseases, as well as increase the growth and resistance of some bacteria. The study did not look specifically at influenza.

Meanwhile, molecular biologist and science journalist Kai Kupferschmidt suggested in Science Magazine that influenza epidemics may become less severe as the planet warms, but outbreaks may become more common, rather than season-dependent. “As epidemics become less severe in a warming climate, the virus is more likely to circulate yearround in many places,” Kupferschmidt wrote.

Fornace, whose research focuses on the impact of environmental change on infectious disease dynamics, tells TIME that “the evidence is still mixed.”

“There is definitely an effect of climatic conditions on spillover events from wildlife (for example, changes in wild bird migration patterns due to climate change),” Fornace says. “There are also some potential effects on the pathogen biology, such as survival and transmission rates under different environmental conditions.”

But the clearest effects may be seen in how people alter their behaviors in relation to the changing climate.

“We know that people spend different amounts of time indoors and have different movement patterns with different weather conditions,” Fornace says. “We have also seen major impacts of climate change across the region on health systems, particularly in response to extreme weather events where it can be difficult to continue to deploy routine health measures,” such as vaccinations.

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Dr. Paul Tambyah, former president of the Asia Pacific Society of Clinical Microbiology and Infection, tells TIME that the emergence of new virus strains or variants is likely to be driving the increase in cases.

The World Health Organization’s vaccine recommendations for the 2026 Southern hemisphere flu season included two strains that are different from the 2025 Northern hemisphere flu season. The B/Austria/1359417/2021 (B/Victoria lineage)-like virus was included in both, while the new recommendations are A/Missouri/11/2025 (H1N1)pdm09-like virus and A/Singapore/GP20238/2024 (H3N2)-like virus.

“This suggests that for both H1N1 and H3N2, there are drifted (or mutated) influenza strains circulating, which we may not be immune to if we got the previous flu vaccine or were infected in the last flu wave,” Tambyah says. “Hence the rise in flu cases in many parts of the world.”

In Japan, health officials have suggested that the flu epidemic could be caused by commingling factors, including the return of mass tourism after a drop during the COVID-19 pandemic. 

“We are seeing a greater circulation of people, both in Japan and globally, with people taking the virus to new places, which is another factor behind the virus adapting to new environments,” Tsukamoto told SCMP.

It’s not clear that the viral strain impacting Japan, Singapore, and other countries is the same, although Tambyah says it’s likely. Still, he cautions that influenza viruses may not be evolving at a significantly more rapid pace than typical as some have suggested.

“The influenza virus has always had the ability to evolve rapidly to avoid immunity from either disease or vaccination,” Tambyah says. “Even the ‘normal’ pace of influenza virus evolution is hard to keep up with.”

But he adds, “there is no evidence that this is happening any faster or slower.”

Tambyah says scientists are closely watching the evolution of H5 and H7 or H9, subtypes of influenza A virus, in birds and animals.

“The big concern is if a totally new strain of influenza emerges other than H1, H3, or Flu B, to which we have very little immunity,” Tambyah says. “This would lead to a pandemic like the H2N2 Pandemic of 1957.” Also known as the “Asian Flu” pandemic because of where it originated, that pandemic nearly 70 years ago killed an estimated 1.1 million people worldwide, including some 116,000 in the U.S., according to the CDC.

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Ramanan Laxminarayan, an economist and epidemiologist who founded D.C.-based public health research organization One Health Trust, tells TIME that vaccination is particularly important for the elderly and other high risk populations. But he cautions, “this approach is dependent on our being able to predict the strains that will cause the following year’s outbreak and include these in the vaccine.”

“In the longer term, a universal flu vaccine that works across many flu strains and will only have to be taken once in a decade would be the long-term solution,” Laxminarayan adds, “but we are yet to get there.”



As the Flu Surges in Asia, Could Getting Sick Year-Round Be the New Normal?