Medical staff collects swab samples for workers from China Railway Engineering Group for novel coronavirus nucleic acid tests on the reconstruction project site of Beijing Fengtai Railway Station, on April 24, 2020. [Photo by Zou Hong/chinadaily.com.cn]

Although efforts are underway to trace the source of the virus for the latest COVID-19 outbreak in Beijing, and initial analysis showed the virus strain was from Europe, the Beijing strain is older than existing strains in prevalence in Europe at present, according to a researcher at China CDC.

"There may be different possibilities for importing of the virus," Zhang Yong, assistant director of China CDC's National Institute for Viral Disease Control and Prevention, said in a report published by the National Supervisory Commission.

"The virus may have lurked in imported frozen food, so it did not evolve during transport from overseas due to the freezing conditions," he said.

Another possibility is the virus was lurking in dark and humid conditions such as Xinfadi wholesale market and was suddenly exposed to humans and caused the outbreak, he said.

Following the report of the outbreak on June 11, experts from the institute have collected hundreds of virus samples from Xinfadi, many testing positive. Some cutting boards for salmon sold in the market tested positive for the novel coronavirus, authorities in Beijing had previously said.

"The fact a number of samples collected from the market tested positive for the virus indicates the virus has remained in the market for quite a while," Zhang said. "However, we need to find more evidence."