Flu Cases Surge Early in Japan, Sparking Global Health Concerns

Flu cases rise early in Japan, raising global health concerns

By Rachel Fieldhouse & Nature Review

Japanese health authorities have declared a flu epidemic, with thousands of people infected with the respiratory virus. The number of infections is unusual for this time of year, researchers say, and could cause outbreaks in countries preparing for winter in Asia and Europe – although it is unlikely to turn into a global pandemic.

As of October 10, 6,013 cases of influenza virus have been reported in Japan. More than 100 schools closed and nearly half of the 287 people hospitalized with flu in September were children aged 14 or younger. The Ministry of Health, Labor and Welfare declared a nationwide epidemic on October 3. Outbreaks are classified as epidemics when the number of infections is higher than expected in a given area over a given period of time.

Influenza virus outbreaks tend to occur seasonally each year, primarily in winter in countries with temperate climates. In Japan, this usually happens around the end of November. This year, the increase in the number of people being treated for flu started five weeks earlier than usual, says Vinod Balasubramaniam, a molecular virologist at Monash University Malaysia in Subang Jaya.

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In Japan, the flu season has started early in recent years, but not this early, says Ian Barr, a researcher and deputy director of the World Health Organization Collaborating Center for Influenza Reference and Research, based in Melbourne, Australia. “You might see cases in October, but not epidemic-type numbers,” Barr says.

The increase in international travel since the COVID-19 pandemic is one factor that could be behind the early start of the flu season, says Balasubramaniam. Other factors include climate change and lack of exposure to the circulating virus, especially for older people and young children.

Information on the strains circulating in Japan is not yet available, but Barr says the outbreaks there may be caused by a strain of influenza A, called H3N2, that has emerged in Australia and New Zealand in the past two months, coinciding with the end of winter in the southern hemisphere. Australians are traveling to Japan in large numbers, he says, meaning there is a greater chance of the virus being transmitted between hemispheres.

Not an exception

Other countries, including Malaysia, also experienced early flu seasons this year, dominated by the H3N2 strain, says Balasubramaniam. About 6,000 students across Malaysia have been infected and some schools have closed, according to Reuters. Australia and New Zealand also saw early spikes in flu cases, with various strains dominating outbreaks, Barr says.

The current flu outbreak in Japan is unlikely to turn into a global pandemic, he says, given that countries in the southern hemisphere are now entering warmer seasons, which reduce the spread of the virus. “But it could very well come out of Japan and spread to other neighboring countries, or to countries on routes from Japan,” Barr says. Countries that are about to enter winter will be more susceptible to epidemics.

This article is reproduced with permission and has been published for the first time October 14, 2025.

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