Beijing Says International Mail Is Possible Culprit in Omicron Case

Tests for more than 16,500 people identified as potentially being exposed to the patient came back negative, Pang Xinghuo, deputy director of the city’s municipal Center for Disease Control and Prevention, said at a press conference on Monday.

Location data showed that the patient, who had no recent travel history, hadn’t shared space with any other known Omicron patients or their contacts, or anyone who had recently returned from abroad, Ms. Pang said in comments reported by Chinese state media.

However, traces of the Omicron variant were found in samples collected from a piece of mail sent from Canada that patient had touched, Ms. Pang said, adding that the mail “could not be ruled out” as the source of the exposure.

China has previously highlighted mail as a possible source of Covid-19 infections. In November, the country’s State Postal Bureau urged employees to be on high alert in handling mail and letters both from abroad and from high-risk areas inside China. It said at the time that Covid-19 had been recently detected from materials shipped into the country from abroad, but didn’t provide details.

The Postal Bureau also repeated earlier assertions that frozen-food imports were another likely vector for Covid-19 infections, despite widespread skepticism outside China. The U.S., the European Union and several other governments have said they see no significant risk that the virus is spreading through the cold chain.

Covid-19 primarily spreads through the air, hitching a ride on tiny water droplets expelled when people talk or cough. The World Health Organization says it is possible for people to become infected after touching contaminated surfaces or objects, but researchers generally believe that risk is very low.

Ben Cowling, chair professor of epidemiology at the University of Hong Kong, said he wouldn’t rule the possibility out but thinks it is fairly unlikely the new Beijing case came from the mail.

“Viruses can remain viable on surfaces for a while at cold temperatures, but transmission via contaminated surfaces is not a common route of transmission,” he said.

Martin Hibberd, professor of emerging infectious diseases at the London School of Hygiene and Tropical Medicine, said that while surface transmission of the virus can happen over very short time periods—touching a doorknob within minutes of an infected person, for example—evidence for longer-term transmission is weak. “The evidence [of surface transmission] is so small, and the fact that it’s not in that 10-minute window, it’s hours and probably days, it seems very unlikely,” he said.

Local spread of the Omicron variant in China was first reported less than a week ago in the port city of Tianjin, which neighbors Beijing, and has since spread to other Chinese cities.

Ms. Pang, the Beijing health official, said the Omicron variant was detected in 10 samples collected from a piece of mail handled by the patient on Jan. 11, four days after it was sent from Canada, by way of the U.S. and Hong Kong. The samples were collected from the exterior of the mail as well as the papers inside, she said.

Health workers also tracked down another piece of mail delivered to a different location from the same sender, and found Covid-19 virus inside and on the cover as well, according to Ms. Pang.

Ms. Pang said gene sequencing showed that the virus that caused the Beijing Omicron infection is more similar to strains found in North America and Singapore than those that caused the Omicron cases elsewhere in China or any previous Covid-19 infections in Beijing—yet another reason why the mail was the suspect.

Health authorities didn’t say whether the Omicron samples from the mail were a genetic match with the virus that infected the Beijing patient.

—Denise Roland contributed to this article.

Write to Wenxin Fan at Wenxin.Fan@wsj.com