The number of new influenza cases reported at designated medical institutions in Japan has surged 57% in the past week, health ministry data showed Friday, marking an unusual trend for a virus usually prevalent during winter.

Health experts attribute the prolonged spread of infections to a combination of factors, including low immunity stemming from a fall in new flu cases when government coronavirus measures were in place.

Approximately 5,000 designated medical institutions across the country reported a total of 34,665 new cases in the week from Sept. 11, up from 22,111 the previous week, according to the data.

Flu patients averaged 7.03 per institution, with seven of Japan's 47 prefectures, including Tokyo, seeing more than 10 per institution, the level regarded as a benchmark to be cautious about a potential influenza outbreak.

Despite the flu season in Japan usually ending in spring, the weekly average number of cases per medical facility has continued to exceed typical levels well into summer this year, even increasing from late August.

The number of new coronavirus infections reported by designated medical institutions during the same week from Sept. 11 stood at 86,510, averaging 17.54 people per institution, down 13 percent from the previous week.

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