Tokyo, Jan 21 (IANS): The average number of flu patients per medical institution in Japan in the week through January 15 came to 7.37, signaling the beginning of an epidemic, a health ministry survey said.

About 5,000 medical institutions in all of Japan's 47 prefectures reported 36,388 cases of seasonal flu during the seven-day period, the National Institute of Infectious Diseases said.

The nationwide per-institution figure was more than 1.5 times that of the previous week, while the tally of influenza patients across the country is estimated to stand at about 257,000, reports Xinhua news agency.

By prefecture, the per-hospital number was the highest in Okinawa at 33.23.

The number was above 10 in seven other prefectures, including Fukuoka at 16.96, Miyazaki at 16.63 and Saga at 15.79.

A reading above 10 suggests a major spread of seasonal flu could occur within four weeks, while a reading above 30 signals that a major spread is suspected to have occurred.