The health ministry issued a warning Wednesday as the number of influenza patients per medical institution reached 11.17 in the week through Dec. 30.
The figure was up by 3.12 from the preceding week, according to reports from about 5,000 regularly monitored medical institutions, suggesting the possibility of an outbreak within four weeks.
Based on the reports, the total number of flu patients at medical institutions across the nation in the reporting week was estimated at some 446,000, the ministry said. The ministry issues warnings when the number of flu patients per institution rises above 10.
By prefecture, the number of patients per institution was the highest in Hokkaido, at 32.07, followed by Aichi, 30.45, Gifu, 20.33, Kumamoto, 14.53 and Mie, 13.68.
Of the flu viruses detected in patients in the last five weeks, type A, which spread globally as a new strain in 2009, accounted for the largest share, followed by the type-A Hong Kong variant and type B.