On the basis of substantially increased hospitalizations, the H1N1 swine flu outbreak was declared an epidemic by the National Institute of Infectious Diseases on Friday.

According to the NIID, the average number of flu patients per hospital during the week of Aug. 10 to 16 was 1.69. A number of 1.00 or above signals an epidemic.

Most of the flu patients are believed to have the H1N1 strain of type-A flu, the institute said, adding it estimates their number to be about 110,000.