From March, genetic mutations of the original COVID-19 virus may have spread undetected, and from late June may have spread nationwide, a new analysis of domestic infections indicates.
The National Institute of Infectious Diseases (NIID) collected roughly 3,700 samples from patients confirmed infected in Japan and other cases through July 16.
Since the genes of the new coronavirus mutate little by little through infecting humans, the institute studied how viruses obtained from one patient and another were similar by comparing their genomes.
In contrast to the European strain of the coronavirus that is believed to have caused the outbreak in Japan from March, viruses whose genes are presumed to have mutated for the next three months were found in the middle of June, according to the results of the analysis announced on Aug. 5.
The researchers later found viruses that possessed the same characteristics in hundreds of samples across Japan, though a virus linking them and the European strain of coronavirus has not yet turned up in their samples.
The NIID raised the possibility that the viruses may have been linked by patients who did not display any coronavirus symptoms.
As Japan began restarting its economic activities, some mutated forms of the virus may have continued spreading, not only in Tokyo, but also in other areas across the nation after late June, according to the analysis.
But the NIID said it could not determine exactly how far the viruses might have spread.
It is not possible to judge how widely the virus can infect others and how easily infected patients develop serious symptoms based only on genome information.
To do those, the NIID's analysis advised that the virus “should be considered comprehensively” while checking findings of patients and investigating the disease.