The clue leading to the international parcel that first brought the Omicron strain of the coronavirus to Beijing came from the infected woman's mother. It has played an essential role during the epidemiological investigation.

Ren Jiangchu, director of Haidian district's Center for Disease Control and Prevention, told Beijing News that the patient, who had not traveled outside the city, was pretty sure she had not had any close contacts with foreigners within the last 14 days.

However, her mother observed that the woman had touched international mail.

"This provided us an important clue," Ren said.

"Usually, people throw away parcel wrappings after opening them, but this international mail was about work and was sorted at the woman's workplace. This has helped us find this mail quickly."

Even though the mail had been received for many days, the authorities were able to detect the virus on the sample. Other international parcels from the same source were later found with the virus.

"All of those clues are important for virus source tracing. Otherwise, the investigation would take much longer," Ren said.

On Saturday, Beijing reported an infection whose genes were similar to strains detected in North America and Singapore.

According to the municipal government, the patient had received a package sen from Canada on Jan 7. It had passed through Beijing on Jan 11 via the United States and Hong Kong.

The patient had handled the outer surface of the package and the first page of an interior document.

The State Post Bureau ordered measures to ventilate and disinfect sites where the contaminated items had been.