Flu returns sooner than expected: Japan faces one of its earliest nationwide outbreaks — is this the new normal?

The flu season began in Japan earlier than usual. Local media cited Japan's health ministry as saying on October 3 that the country experienced the second-earliest flu outbreak in the past 20 years.

Setting aside 2023, as flu cases were high throughout that year, 2009 had the earliest start for the flu season on record, the Japan Times reported.

According to Time magazine, more than 4,000 people in Japan were treated for influenza between September 22 and September 28. The number of patients treated for influenza increased to over 6,000 people from September 29 to October 5, according to the health ministry's data.

"Influenza season has begun," Japan's Ministry of Health posted on X on October 3.

But not just in Japan, a surge of influenza cases have been witnessed in countries across Asia. This prompted some medical experts to warn that getting sick more easily throughout the year may be the new reality.

Flu vs common cold

Seasonal influenza (the flu) is an infection caused by any of the four types of influenza viruses A, B, C, and D, with influenza A and B causing the most common seasonal outbreaks in humans.

Symptoms of influenza include acute onset of fever, cough, sore throat, body aches and fatigue.

The flu and the common cold share many similar symptoms. However, the common cold can be caused by several different viruses, including rhinoviruses, according to the US Centers for Disease Control and Prevention (CDC).

Influenza viruses vs seasonal influenza (flu) virus

Notably, influenza viruses are the broad category for all viruses that cause the flu, while 'seasonal influenza (flu) virus' refers specifically to the strains (primarily A and B) that cause annual, predictable outbreaks, often in winter.

When are flu/influenza cases higher

Influenza cases are typically higher between December and March and between May and August, according to Singapore’s Communicable Diseases Agency.

Meanwhile, the World Health Organization (WHO) says that in temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year, causing outbreaks more irregularly.

In the United States, between 1982-1983 and 2023-2024, flu activity most often peaked in February (17 seasons), followed by December (7 seasons), January (6 seasons) and March (6 seasons).

Is flu becoming the new normal?

Yoko Tsukamoto, a professor at the Health Sciences University of Hokkaido, was quoted by the South China Morning Post as saying, “The flu season has started really early this year, but in the changing global environment, this might become a more common scenario."

Meanwhile, Dr. Kimberly Fornace, an associate professor at the National University of Singapore’s school of public health, told 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.

Besides this, 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.

Why are influenza cases more frequent now?

Experts warned that the virus may be evolving faster and spreading more easily or earlier than typical in some regions. Moreover, an early and rapid spread suggests the influenza virus is adapting at an unprecedented pace.

Emerging research indicates the virus is not only spreading more efficiently but may also be developing resistance to conventional treatments.

Tsukamoto told This Week in Asia that these changes could help explain why Japan was experiencing its second-earliest flu outbreak in two decades.

“We are seeing this resistance in Japan, but it is also being reported in other parts of the world as well,” she said.

Scientists and experts also point to a "confluence of factors" behind the surge. Mass tourism in the post-pandemic era could be one reason which has accelerated the movement of people – and viruses – across borders.

“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 said.

Moreover, “Influenza may transmit more easily when people are spending most of their time indoors during winter,” Dr. Kimberly Fornace told TIME.