On the basis of substantially increased hospitalizations, the H1N1 swine flu outbreak was declared an epidemic by the National Institute of Infectious Diseases on Aug. 21.

A supermarket in Adachi Ward, Tokyo, increases its stock of masks and disinfectant solutions in preparation for the spread of swine flu Aug. 24. KYODO PHOTO

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.

Since early July, swine flu patients have increased in number by 1.7 times each week. Three recently died within the span of five days.

The NIID said the number of patients per hospital exceeded 1.00 in hospitals surveyed by 264 public health centers in 44 prefectures, excluding Hokkaido, Toyama and Kumamoto, indicating the epidemic is increasing in area and level, it said.

With the first deaths in Japan from the H1N1 virus, there is a renewed sense of anxiety on the streets.

But according to an infectious diseases expert, what may seem a sudden lethal outbreak is only the expected return of the new flu by fall, which the nation has prepared for by stockpiling the virus-fighting drugs Tamiflu and Relenza.

"This is only the beginning of the first big wave, and it will continue to spread into September and October," said Norio Sugaya, an infectious diseases expert and head of the Pediatrics Department at Keiyu Hospital in Kanagawa Prefecture.

"It came slightly earlier than expected, but it's hardly a surprise," he added.