Japan is grappling with a sharp rise in seasonal influenza cases, prompting health authorities to urge people to take heightened precautions as the year-end holiday period approaches.

Data that the health ministry released Friday highlights a troubling spike in infections. From Dec. 9 to 15, there were 94,259 cases of the flu reported across some 5,000 hospitals and clinics nationwide.

This brings the estimated total of cases nationwide to 718,000, more than double the figure recorded the previous week.

The national average of reported cases per medical institution jumped to 19.06, up from 9.03 the week before. All 47 prefectures saw increases, with Oita leading, at 37.22 cases per medical facility, followed by Fukuoka at 35.40, Kagoshima at 29.00, Ehime at 28.90, and Saga at 28.55.

Oita and Fukuoka surpassed the warning threshold of 30 cases per institution, while 40 prefectures exceeded the alert level of 10.

Tokyo has also seen a steep rise in infections, recording 17.36 cases per medical facility as of Dec. 15, a sharp increase from 8.19 the prior week. This figure breaches the city’s advisory threshold of 10, signaling a high likelihood of a large-scale outbreak within the next four weeks.

The flu has even reached the Imperial Household. Princess Aiko contracted the virus and is expected to remain absent from official duties until at least Friday, the Imperial Household Agency announced Monday.

Outbreaks in schools and care facilities are intensifying, with cluster outbreaks doubling to 211 in Tokyo from Dec. 9 to 15 compared to the week before. Nationwide, 2,759 schools and other institutions implemented partial or full closures from Dec. 9 to 15, also twice the previous week’s total.

Hospitalizations have risen in tandem, with 1,316 patients nationwide admitted for influenza during the same period, up from 628. Of these, 58 required intensive care, up from 43.

The rise in hospitalizations has prompted concerns among officials about potential burdens on the health care system, as the flu outbreak comes in combination with a rise in COVID-19 cases.

“We predict high case counts this season because people were less exposed to the flu virus during the pandemic and so lack immunity,” Tadashi Ishida, head of the influenza panel at the Japanese Association for Infectious Diseases, told NHK.

Dr. Norio Sugaya, an honorary adviser at Keiyu Hospital, noted a return to prepandemic flu patterns.

“Before COVID-19, flu cases typically surpassed alert thresholds in late December and peaked in January,” he said in an interview with the Asahi Shimbun, adding that stringent COVID-19 measures between December 2020 and spring 2022 largely suppressed influenza outbreaks.

In contrast, flu cases began surging unusually early in September 2023, with waves of infection continuing into May 2024. During this period, an estimated 18 million people — 1.8 times the usual number — were infected.

With restrictions now lifted, both Ishida and Sugaya expect cases to peak in January, consistent with current ministry and prefectural data.

In response to the outbreak, health officials are urging the public to adopt rigorous preventive measures, including regular handwashing, use of hand sanitizers, and practicing proper cough etiquette.

Vaccination also remains the most effective way to prevent severe flu symptoms, particularly among vulnerable groups.