Influenza outbreaks started in Japan in late September, earlier than usual, partly because of increases in cross-border travel.

According to the health ministry, the number of flu patients reported by regularly monitored medical institutions across the country in the week to Oct. 26 stood at 6.29 per facility. By prefecture, Okinawa topped the list, with 19.40, followed by three prefectures neighboring Tokyo ¡ª Kanagawa, with 11.88, Chiba, with 11.82, and Saitama, with 11.73.

The number began to exceed 1.0 in late September ¡ª the second earliest increase in 20 years excluding the 2023-24 season, when outbreaks continued yearlong. A reading of 1.0 or higher suggests the start of a flu season.

Flu patients fell markedly in and after 2020, thanks to measures against COVID-19.

But flu outbreaks spread across the country for the first time in three years in late 2022, and the season continued even after summer 2024. In December that year, the number of patients hit the highest level since the current survey method was adopted in April 1999.

As a reason for the surge in patients, Atsuo Hamada, a professor at Tokyo Medical University, pointed to a decline in people¡¯s immunity against influenza after the long absence of outbreaks amid the COVID-19 pandemic, during which strong infection prevention measures also curbed the spread of other diseases.

As to the ongoing outbreaks, Hamada cited the possibility that infections through droplets grew due to increased incidence of people gathering in air-conditioned locations amid the lingering summer heat.

The Japan Meteorological Agency said in early October that the nationwide average temperature in September was the third highest for the month since statistics began in 1898.

The flu test positivity rate has remained high in some areas in the world, topping 30% in Southeast Asian nations, according to a recent report from the World Health Organization.

A surge in foreign visitors may have led to the current flu outbreaks. The number of visitors to Japan in September totaled some 3.26 million, surpassing 3 million for the first time for the month, according to Japan National Tourism Organization data.

Basic measures ¡ª such as washing hands, wearing masks in crowds and frequent ventilation ¡ª are effective in preventing infection.

Vaccination, including intranasal sprays available for those age 2 to 18, is believed to be effective for six months. Hamada urged people to get vaccinated early if possible.