A wharf in Yantian port zone of Shenzhen port in south China's Guangdong Province. File Photo: Xinhua

China's postal administration on Monday vowed to "build a barrier" against COVID-19 cases imported through international express mail, after at least two Omicron cases, one in Beijing and another one in Shenzhen, were suspected of tracking back to packages from North America.

Virologists warned of the high risk of packages from overseas causing domestic COVID-19 flare-ups, when the epidemic overseas is running high, as contaminated packages from epidemic-rampant countries may spread the virus to people with weak immunity through viral particles floating in the air, or people without proper protective gear.

A patient from Shenzhen, who was confirmed to be infected with Omicron on Sunday, is likely to have contracted it after being exposed to contaminated goods from overseas, Shenzhen's health authorities said at Monday's press conference.

The patient, whose work involved handling overseas frozen reagents for collection and delivery, had touched and opened packages from North America without using personal protective gear, according to the press conference, adding that the virus' origin is still under probe. 

On the same day, Beijing health authorities revealed that a patient from Beijing who was found to have Omicron on Saturday had received a piece of international mail on January 11, which was sent from Toronto, Canada on January 7 and then transferred via the US and the Hong Kong Special Administrative Region to Beijing. The Omicron variant was detected on the outer package, internal surface and paper of the mail. 

The Omicron strain of the patient in Beijing is similar to those of North America and Singapore in December 2021, Pang Xinghuo, deputy director of the Beijing Center for Disease Control and Prevention (CDC), said at Monday's press conference. The possibility that the Beijing case was infected through the international mail cannot be ruled out, Pang said.

The Beijing CDC encouraged people to reduce purchasing overseas commodities as the epidemic overseas is rampant. 

Yang Zhanqiu, a Wuhan-based virologist, told the Global Times on Monday that as the overseas epidemic is still rampant, chances are high that those contaminated international packages sent to China would cause domestic outbreaks. Those packages could carry or be contaminated with virus at the departure point or during the delivery process. 

The confirmed patient in Beijing had contact with the outer package of the document and the front page of the document inside the mail. Omicron was detected in two samples of the outer surface of the mail package, two samples of the internal surface of the package as well as eight samples of the paper inside, according to the Beijing CDC. 

Judging from the released information, Yang said the document delivered from Canada probably had been contaminated by others, or by the person who sent the package, who may be a COVID-19 patient or lived in a COVID-19 environment. Those paper documents are likely to be infected by this person through a cough, or his pen or other contaminated objects. 

Yang explained that a live virus could survive at least few days at room temperature, a month in temperatures around -30 C or a few years in temperatures around -80 C.

The virus on an international package could spread to the respiratory tract of the person in contact with it, through viral particles that were blown by the wind and floated in the air. The chance of packages or papers spreading the virus to people via aerosol transmission is low, though, Yang noted. 

Yang warned people to take precautions when receiving mail or goods from overseas by wearing masks and gloves and disinfecting and disposing of the packages properly.

As the Spring Festival holidays and the Beijing Winter Olympics approach, China's State Post Bureau and the Beijing postal administration vowed to optimize anti-epidemic measures on packages from overseas or domestic epidemic-affected regions, and strictly prevent the international packages from causing domestic COVID-19 flare-ups.