Flu Cases Surge Early in Japan, Sparking Global Health Concerns

Flu Cases Surge Early in Japan, Sparking Global Health Concerns

By Rachel Fieldhouse & Nature magazine

Japan’s health authorities have declared an influenza 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 seed outbreaks in countries that are heading into winter in Asia and Europe — although it is unlikely to become a global pandemic.

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

Outbreaks of influenza virus tend to occur seasonally each year, predominantly in winter across countries with temperate climates. In Japan, that usually occurs around the end of November. This year, the increase in 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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Japan has had early starts to the flu season in the past few years, but not this early, says Ian Barr, a researcher and deputy director of the World Health Organization Collaborating Centre for Reference and Research on Influenza, who is based in Melbourne, Australia. “You might see cases in October, but not epidemic-type numbers,” says Barr.

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

Information about which strains are circulating in Japan has not yet emerged, but Barr says the outbreaks there could be caused by a strain of influenza A, called H3N2, that surged in Australia and New Zealand over the past two months, coinciding with the end of winter in the Southern Hemisphere. People from Australia are travelling to Japan in huge numbers, he says, meaning there are more chances for virus transmission between hemispheres.

Not an outlier

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

Japan’s current influenza epidemic is unlikely to become a global pandemic, he says, given that countries in the Southern Hemisphere are now entering warmer seasons, which reduce viral spread. “But it could well go out from Japan and seed into other countries nearby, or countries that are on the travel routes from Japan,” Barr says. Nations that are about to enter winter will be more susceptible to outbreaks.

This article is reproduced with permission and was first published on October 14, 2025.

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