The influenza season is in full swing in Japan after an earlier-than-usual start this year.

The season typically takes place between December and March, but health ministry data for the Sept. 4-10 period, released Friday, showed that the flu is already quickly spreading across the country, leading to school closures in many prefectures.

The typical case count that signals the start of influenza season is one per designated medical institution. However, based on reports from roughly 5,000 medical institutions across the country, the national average for the week through Sunday was 4.48 per institution, jumping from the previous week’s average of 2.56.