TOKYO: Japan’s National Institute of Infectious Diseases has said in a study report that the novel coronavirus may have continued to spread unnoticed among carriers with mild or no symptoms in Japan after the outbreak from March to April receded once.
The recently released study report suggested that such phenomenon may have led to a resurgence of COVID-19 infections in Japan in and after June, when the country resumed its economic activities.
In the study that ran through July 16, the institute collected the new coronavirus from some 3,700 patients in Japan and analyzed the genome sequence.
As a virus undergoes mutations in the course of infections, the institute deduced how COVID-19 spread in Japan by looking at how the virus changed.
The genome analysis found that the so-called European type of the new coronavirus spread widely in Japan in and after March but its outbreak waned in late May thanks to infection prevention measures.
In mid-June, however, an apparent variant of the European-type coronavirus that is believed to have been created after mutations over three months was found in Tokyo. A virus derived from the variant was found in many other parts of Japan.
As the institute did not find any virus that was in the middle of a mutation process, it pointed to the possibility that the new coronavirus had quietly spread among younger carriers who tend to show no or little symptoms and thus go unnoticed by local public health centers.
In the report, the institute noted that the new coronavirus apparently could not be contained within Tokyo and eventually spread across the country partly because people started making business trips to regional cities after the country reopened its economy.