The CDC said on Wednesday that the virus was found on one parcel from Inner Mongolia that was delivered to Beijing and another sent to Liaoning province.

Two other packages sent and delivered within Inner Mongolia also showed traces of the virus.

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Zhang Liubo, the CDC’s chief expert for disinfection, said no cases of infection were linked to the packages, but deliveries to and from high-risk areas should stop.

“High-risk areas should stop receiving and delivering packages, and couriers should improve how they monitor their health,” Zhang said after a State Council meeting about virus control measures.

“It’s essential to protect the packages during the delivery process. After parcels arrive at the centralised sorting area, the surface of the package should be disinfected, usually with cleaning spray and ultraviolet radiation.”

Courier company ZTO Express said it would fully disinfect its mail delivery trucks and packages.

SF Express said it would step up epidemic prevention measures to ensure the safety of the full delivery chain.

But scientists said it was difficult for the coronavirus to spread via parcels.

Emanuel Goldman, a professor of microbiology at Rutgers University, said transmission was “extremely unlikely” if someone only made contact with a contaminated surface.

“The virus is fragile and dies quickly outside of a human host under normal real-life conditions,” said Goldman, who wrote a commentary on the transmission of the virus for the medical journal The Lancet last autumn. “This is a disease that you get by breathing, not by touching.”

He added that a false positive would have come about from the remnants of RNA, or the building blocks of the virus, even though the virus was no longer capable of infecting people.

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Professor Leo Poon Lit-man, head of the University of Hong Kong’s public health laboratory sciences division in the faculty of medicine, said the virus was unlikely to be transmitted through the mail but it might be worth paying attention to see if the workers handling the parcels were carriers themselves.

“The real concern is, why are there so many packages testing positive? Maybe infected [people] are in the courier company, or in the post office, and they continue to work in these settings, so they contaminate the parcel,” he said.

If they want to reduce the risk, they could wear gloves and a mask and maintain hand hygiene when handling the deliveries, he said.

“We have to be alert … but it’s not a huge threat,” he said.

The country has battled a series of small outbreaks and sporadic cases in dozens of provinces and cities in recent months.