With the influenza season approaching its peak this winter, the number of patients reached about 660,000 nationwide in the week of Jan. 13 to 19, almost doubling the 340,000 tallied the previous week.

The Health, Labor and Welfare Ministry issued a warning last week that the outbreak has reached epidemic proportions and that the peak won’t be reached until early February.

The health ministry said Okinawa Prefecture, followed by Miyazaki, Gifu, Oita and Fukuoka, are the hardest-hit areas.

Close to 700 schools for all age groups were closed from Jan. 13 through 19, and the number is expected to rise.

Although the epidemic has spread mainly in western Japan, it was also expanding in the Kanto region, including Tokyo and Chiba, Kanagawa and Saitama prefectures.

Large numbers of cases appeared in early December in Saitama and then spread to Tokyo, Kanagawa and Chiba in the week of Dec. 16 and expanded through the third week of January, according to the Tokyo Metropolitan Infectious Disease Surveillance Center.

“The epidemic is bound to spread even further in the next few weeks,” said Yoshiyuki Sugishita of the center.

Although the flu peak varies from year to year, it usually occurs between late January and early February. The health ministry said that during last winter, the number of patients increased from 800,000 in the week of Jan. 7 to 13 to 1.4 million the following week and then to 2.14 million in the week of Jan. 21 to 27.

The National Institute of Infectious Diseases said that almost 50 percent of the cases were due to Hong Kong A-type virus, followed by H1N1 swine flu, accounting for about 30 percent.

Sugishita said the reason for the resurgence in swine flu, which became a worldwide epidemic in 2009, has not yet been discovered.

“Influenza is expected to approach its peak in the next week or two, so countermeasures such as not getting close to anyone who has the flu, washing your hands thoroughly and wearing a mask become necessary,” he said.

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