Japanese researchers say they think the coronavirus continued to spread undetected in Japan after the outbreak was thought to have been contained in May.
The National Institute of Infectious Diseases earlier this week released the outcome of its detailed genetic analysis of the coronavirus. The samples were taken from people who contracted the virus in Japan to assess how it had spread across the country.
The researchers say the virus that started to spread in Japan in mid-March is the European strain, which originated in the Chinese city of Wuhan and entered Japan via Europe. The outbreak caused by this strain appeared to be subsiding in Japan in May.
However, cluster infections started to occur across the country from mid-June onwards.
A genetic analysis of virus samples taken from these clusters found they were mutated versions of the European strain.
The researchers say no mutation in the pathogenicity of the virus has been confirmed.
The researchers say the findings suggest that the virus continued to be transmitted through asymptomatic carriers and those with light symptoms and reemerged in Japan in mid-June. They add that the virus may have spread beyond the Tokyo area when people were no longer being urged to refrain from traveling.