Since last year, the cold chain has been seen as an important part of COVID-19's prevention and control. In October 2020, the source of the epidemic in Qingdao also pointed to 2 docks who moved into cold chain sites.

It is worth noting that three of the 71 dockers in the same batch did not wear masks for smoking, and two of them were infected. According to the protection requirements, no one wearing masks was infected.

A team from China's CDC and Qingdao CDC team published a research article in the latest China English edition of China CDC Weekly, which confirmed the possibility of COVID-19 spreading from cold chain to human. In September 2020, two new wharf workers in Qingdao, China were tested for new asymptomatic viral infection after routine screening of throat swabs. Genome sequencing confirmed that two people had been infected with cold packaging products carrying COVID-19. The corresponding authors of the study include Gao Fu, academician of the Chinese Academy of Sciences and director of the Chinese Center for Disease Control and prevention. Covid-19 has been detected in the outer packaging of cold chain seafood imported from abroad since the outbreak of the new crown outbreak. COVID-19 nucleic acid is detected. Several waves of local outbreaks, especially the two outbreaks in Xinfadi, Beijing and Dalian, are considered to be related to the first patient's exposure to cold chain overseas products. Previous epidemiological investigations have proved this. However, due to the researchers' failure to isolate the live virus, the transmission of cold chain product virus to humans cannot be confirmed.

The research team said that the investigation in this study isolated live viruses from cold chain products, which further confirmed the possibility that cold chain products could be transmitted to humans through long-distance transportation.

Novel coronavirus pneumonia was released in October 17th. China's CDC official website has released news that in October 17th last year, China CDC detected the live virus from the positive samples of imported frozen cod from workers' handling of the new crown pneumonia. This is the first time that neocoronavirus has been isolated from the outer packaging of cold chain food in the world, and it is confirmed that exposure to the outer packaging contaminated by neocoronavirus can lead to infection.

From August 11, 2020, Qingdao will conduct nucleic acid testing for high-risk groups, including personnel working in the processing, sales and shipment of frozen seafood every 14 days. Two docks belonging to the same company were screened for COVID-19 positive in September 22, 2020 and September 24th. These cases are the source reports of 151 days in Qingdao.

During the 23 (September 8, 2020) test, 2 asymptomatic docks did not live in high-risk areas, and did not contact COVID-19 infected patients or have a fever of unknown origin. There were no imported cases in the community where they lived, and they had no contact with overseas returnees. During the suspected virus exposure period, both dock workers were exposed to foreign frozen cod unloaded from ship K: they handled frozen cod in two different storage warehouses from 20:00 on September 19 to 6:00 on September 20, 2020. Through a retrospective study, the researchers investigated the contact history and behavior of 71 dock workers transporting k-ship cold chain products when loading frozen cod. The survey found that the two infected dock workers had risk behaviors, including smoking and wearing masks to expose their noses. It is worth noting that the two infected persons took off their masks and smoked, while the non-smokers were not infected.

Since July 2020, about 90 crew members have boarded trawler v. On September 3, 2020, the packaged frozen cod was transferred from trawler V to vessel K. The crew of trawler V took the COD from their cold storage, transferred the cod to ship K by crane, and frozen the COD on ship K (- 18 ℃). The study concluded that, as the crew of the V ship did not accept COVID-19 nucleic acid testing, and no evidence of any K crew member was infected with COVID-19, the COVID-19 contamination source on the frozen food packaging was likely to come from the crew of the trawler V. However, as the infection status of the crew of ship V is unknown, the transmission chain cannot be clearly established at present.

The researchers sequenced the whole genome of 2 case samples and 11 frozen cod samples with positive nucleic acid results. Compared with Wuhan reference strain, there were 12-14 nucleotide mutation sites in the 11 whole genome, including the same 12 nucleotide mutation sites. The researchers initially judged that 2 Wharf workers were infected with COVID-19 by loading and unloading the contaminated frozen cod.

Six high viral load samples collected in frozen cod trays were used to further isolate the virus. Typical coronavirus particles were observed under cryo electron microscope, indicating that the live and infectious COVID-19 was isolated from the samples. The sequencing results also showed that the positive results of virus culture were not caused by pollution. The research team believes that this study confirms that COVID-19 can spread to all countries and regions through international ocean fishery trade, and the way of communication is similar to that of COVID-19 in close contact with polluted families and medical institutions. This has posed a major public health challenge to countries and regions that have blocked COVID-19's local spread.

The researchers proposed that, firstly, the study found that when the contaminated packaging has a high amount of virus, the virus can survive at low temperature for weeks to months (the longest record is from July to September at present), and the virus may still infect employees after being stored in the cold storage for months or years. Secondly, the research shows that sars-cov-2 can survive in humid, low temperature and high salt environment. Therefore, it is necessary to further study the effects of environmental factors such as pH value, salinity, temperature and humidity on virus survival.

The researchers suggest that global cold chain management should be strengthened to prevent COVID-19 contamination in fish and meat production. It is necessary to conduct regular nucleic acid screening for high-risk groups in order to find infected people at an early stage.

They also called on the international community to pay close attention to COVID-19's cold chain dissemination and establish an international response mechanism to prevent COVID-19 from polluting the food in cold chain production, marine fishing and processing, transportation and other links. At the same time, it is necessary to strengthen the health and epidemic prevention knowledge training for employees and relevant professionals engaged in the outer packaging treatment of imported cold chain products, and further improve the formulation of equipment and facilities standards related to the working environment of imported cold chain food and personal protection standards. In addition, the study also recommended that port staff be included in the high-risk group of emergency vaccination.