The health ministry said Wednesday that the number of influenza patients per medical institution reached 11.17 in the week through Dec. 30, exceeding the warning level of 10.

The figure was up by 3.12 from the preceding week, according to reports from about 5,000 regularly monitored medical institutions, which suggests the possibility of an outbreak occurring in four weeks.

Based on the reports, the total number of flu patients at medical institutions across the nation in the reporting week is estimated at some 446,000, the ministry said.

By prefecture, the number of patients per institution was the highest in Hokkaido, at 32.07, followed by Aichi, at 30.45, Gifu, at 20.33, Kumamoto, at 14.53 and Mie, at 13.68.