Japanese researchers found that the novel coronavirus that has transmitted in Japan since June is a variant of a European strain,  the Japan Times reported on Friday.

Researchers from the National Institute of Infectious Diseases collected samples from about 3,700 patients in Japan for the genome sequence study.

They found that the country's outbreak in mid-March was mainly caused by a European strain of the novel coronavirus. The spread of the virus waned in May due to containment measures. However, a month later, a variant of the virus, with a new genetic sequence, appeared centered in Tokyo and spread throughout the country.

In mid-June "an apparent variant of the European type was found in Tokyo that is believed to have emerged after more than three months of mutations," according to the Japan Times. "And a virus derived from that variant was later found in many other parts of Japan."

The research also suggested that the virus reemerged in June and might have been transmitted quietly among younger carriers, with mild or no symptoms, making it hard for public health centers to detect it.

Japan now has over 48,000 infections and reported more than 1,500 new cases in a single day for two consecutive days, according to the data released by NHK News on Sunday.