In the World Health Organization (WHO)-China joint report of COVID-19 origins released in March this year, introduction through cold/food chain products is listed as a "possible" pathway.

In late 2019, the COVID-19 outbreak took place in Wuhan, capital city of central China's Hubei Province. Many of the early cases were found to be associated with the local Huanan Seafood Wholesale Market, where many of the goods sold were frozen.

A COVID-19 resurgence occurred in mid-2020 in Beijing, with all of the infections linked to a wholesale agriculture market called Xinfadi. After several months of analysis, researchers have concluded that imported food via cold chain from high-risk areas overseas may be the origin of the resurgence, warning that the virus could be reintroduced via cold-chain transportation of contaminated items and initiate an outbreak.

In October, China detected and isolated live coronavirus in a package of imported frozen cod during a trace-back investigation into the outbreak in Qingdao in eastern Shandong Province. It was the first time that live virus was isolated from cold-chain food packaging, which further confirmed that the coronavirus can survive on packaging for a long time during cold-chain transportation, signaling that the virus can be carried over long distances across borders.

Other port cities in China such as Tianjin and Dalian also experienced coronavirus transmission traced back to imported cold-chain food.

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Graphics: How is China coping with coronavirus imported through cold chain food?

The WHO-China joint report said the supply chains to Huanan market included cold-chain products and animal products from 20 countries, including those where samples have been reported as positive for SARS-CoV-2 – the virus that caused the COVID-19 pandemic – before the end of 2019 and those where close relatives of SARS-CoV-2 are found.

The virus "has been found to persist in conditions found in frozen food, packaging and cold-chain products," said the report.

"Index cases in recent outbreaks in China have been linked to the cold chain; the virus has been found on packages and products from other countries that supply China with cold-chain products, indicating that it can be carried long distances on cold-chain products," it said.

"There is good evidence from the outbreaks in Beijing, Qingdao and Dalian that this cold chain plays a role," said WHO team member and zoologist Peter Daszak in an interview with CGTN in this February. "So that needs to be tested. Go out and find out more information."

Wang Chen, director of the National Center for Respiratory Medicine, noted at a press conference in July that cold chain in infectious pathways for respiratory disease is a new phenomenon that deserves attention.

In terms of epidemiology, associations have been found between the cold chain and the COVID-19 cases, which can be seen from the COVID-19 flare-ups in Beijing's Xinfadi market and Qingdao. The infected people were exposed to the cold chain without other possible sources of the virus, said Wang.

In terms of detection of the nucleic acid of the virus, positive results have been found on cold-chain goods, he said. During the outbreak in Qingdao, in particular, the RNA of SARS-CoV-2 was detected on the surface of cold-chain goods and the virus was isolated, proving the fact that living virus existed on the surface of cold-chain goods.

"Given the two points above, the chain of evidence for the transmission has become relatively complete," said Wang.

He added that there's circumstantial evidence for the exploration of the scientific issue, which is that the virus can survive and remain infectious for a longer period in a cold and relatively dry environment.

"It can even survive for 21 days in winter. Therefore, when the virus emerges in a place, and if the cold-chain products get the virus, the virus can be transmitted from one place to another in a low temperature during transportation, leading to cross-regional transmission accordingly."

Chinese experts suggested cold-chain exports should be one of the focuses in the WHO's second-phase study of COVID-19 origins.

"The next step in tracing the origins of the new coronavirus responsible for the ongoing pandemic should be directed at countries where the virus had been found in animals, the environment and human samples earlier than the Wuhan outbreak in 2019, and the inquiry should also take in countries that exported cold-chain products to the city's Huanan Seafood Market," reported South China Morning Post citing Liang Wannian, leader of the Chinese side of the China-WHO joint team.

"To further trace origins of the virus, we suggest that the cold chain should be taken as a key clue," Wang echoed. "Given the circumstances of international trade, with the exchange of people and goods around the world, the items-to-human transmission in the cold-chain environment has made pathogen transmission more complicated, which is particularly worthy of investigation and studies."