NHK has learned that researchers at Keio University have detected a strain of coronavirus that is different from others currently spreading in Japan.
A team led by Professor Kosaki Kenjiro of the Keio University School of Medicine has been analyzing the genes of coronavirus samples from patients at 13 hospitals across the country.
Last November, the researchers found that a patient in the Kanto region, which encompasses Tokyo, had a strain called "20C," which has mainly been spreading on the US West Coast.
The patient had not traveled abroad, which suggests the virus was community-acquired.
The current outbreak in Japan mainly consists of two types of the coronavirus that remained after the first wave last year. The 20C strain has not been detected since May last year, except at quarantine stations.
Japanese quarantine officials are stepping up measures to keep out variants found in Britain and South Africa that are said to be highly communicable.
The team says finding the 20C strain underlines the risk of new variants being brought in from overseas.
Kosaki says genetic analyses show quarantine measures have been working, but that cases of the virus are still coming in from abroad.
He stresses the need to analyze virus genes nationwide and to grasp the infection situation accurately and quickly.