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Japan has seen very few influenza patients amid the novel coronavirus pandemic, possibly because the coronavirus has prevented infection with flu viruses, on top of people washing hands and wearing masks in the fight against the pandemic, experts say.

The flu season in Japan usually starts around November and December when the weekly average number of patients per medical institution exceeds one, and peaks out between January and February.

This year, however, there were only three to 32 flu cases each week in September and October, or 0.1% to 1% of the levels seen a year earlier, and 139 cases in the first four weeks of November, or 0.24%. In the week to Dec. 6, the per-institution average stood at 0.01, far below the flu season declaration level, according to the health ministry, which receives flu reports from some 5,000 hospitals and clinics across the country.

“The coronavirus is so highly infectious that people are less susceptible to flu viruses in areas where the novel virus is raging,” in a phenomenon called viral interference, Tetsuo Nakayama, specially appointed clinical virology professor at Kitasato University’s Omura Satoshi Memorial Institute, said.

Cells infected with the coronavirus secrete interferons, proteins that play important roles in the immune system, and block infection with other viruses, he explained.

For instance, an epidemic of the respiratory syncytial virus, which causes cold-like symptoms, ends as a flu virus starts spreading, Nakayama noted.

He also pointed out that the coronavirus interference is believed to have prevented flu viruses from spreading in the Southern Hemisphere in the winter months of May and June.

But a health ministry official called for continued vigilance against flu, noting that there is no evidence of viral interference at the moment.

“Basic infection preventive measures such as washing hands and gargling should be conducted thoroughly,” the official urged.

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