The central government on Friday said the average number of influenza patients at thousands of designated medical institutions nationwide had hit warning levels at their fastest pace in 10 years.
Across roughly 5,000 institutions, 166,690 patients had been reported in the week through Dec. 10, averaging 33.72 people per facility and surpassing the warning level of 30, according to the Ministry of Health, Labor and Welfare.
The National Institute of Infectious Diseases estimated that the number of patients nationwide totaled around 1,118,000 during the same period.
The rapid spread reflects lowered influenza immunity after cases had dropped in recent years amid anti-infection measures implemented against the coronavirus pandemic.
Health experts also say that the influenza virus is spreading about a month earlier than normal.
Coronavirus cases have also been increasing for the third consecutive week, authorities said. Experts have called for caution amid fears both viruses could spread further as year-end and New Year's social gatherings take place in Japan.
School and class-specific closures have been required at 6,382 educational facilities nationwide in the week through Sunday, the ministry said.
While influenza outbreaks typically occur in the winter and the end of spring, this year saw an unusual increase in cases from August.
The ministry cautioned against the flu in October when cases exceeded the advisory level for that month of 10 people per institution.
"Individual measures for preventing infection are the same as those for COVID-19, including getting vaccinated, wearing masks, and avoiding crowded places," said Nobuhiko Okabe, head of the Kawasaki City Institute for Public Health.