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The National Institute of Infectious Diseases has said in a study that the coronavirus may have continued to spread unnoticed through carriers with mild or no symptoms after the outbreak from March to April receded once.

The recently released study suggested that such a phenomenon may have led to the resurgence of COVID-19 infections in and after June, when Japan began resuming economic activities.

In the study, which ran through July 16, the institute collected coronavirus samples from some 3,700 patients and analyzed the genome sequence.

Because a virus undergoes mutations during the course of infection, the institute deduced how COVID-19 spread by looking at how the virus changed. The genome analysis found that the so-called European type of the virus spread widely in Japan in and after March but waned in late May thanks to preventive measures.

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

As the institute did not find any virus that was in the midst of the mutation process, it pointed to the possibility that the coronavirus quietly spread among younger carriers, who tend to show no or little symptoms, and thus went unnoticed by public health centers.

In the report, the institute noted that the coronavirus apparently could not be contained within Tokyo and eventually spread across the country, partly because people started making business trips after the economy reopened.

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