HONG KONG — China has warned of Covid-19 risks from the handling of imported cold-chain foods and stepped up its control measures following studies showing strong links to local outbreaks in Beijing and Qingdao.

The warning came after epidemiological investigators pinned down the outbreak in Beijing in June, involving 335 cases, to imported salmon sold from a booth in a wholesale food market.

Investigators published their findings in the National Science Review on Friday (Oct 23), saying the result was “particularly important” for countries where community transmissions were contained or suppressed and that a Covid-19 outbreak could start from the cold-chain transport of contaminated items.

This month, the Chinese Centre for Disease Control and Prevention detected and isolated living Sars-CoV-2, the virus that causes Covid-19, on the outer packaging of frozen cod handled by two port workers in Qingdao who became infected in September.

The two workers had a CAT scan, triggering a hospital cluster exposing other patients using the same scan room to the virus. In addition to the workers, a further 12 people in the city were infected.

On Tuesday, a government task force for Covid-19 control issued a guideline to step up Covid-19 prevention measures for people handling imported frozen food, citing a possibility of “long-distance, cross-border import with cold-chain food as carrier”.

Beijing has launched a compulsory digital platform to track and trace imported frozen food.

On June 11, a 52-year-old man was the first Covid-19 case after Beijing had no new cases for 56 days. The Xinfadi market he visited was identified as the source of infection after all close contacts and samples from the man’s environment tested negative for the virus — except two from the market.

Through citywide testing the investigators found all cases were market employees, visitors or their close contacts, suggesting the market was the single source.

The scope was narrowed to the basement trading floor with significantly higher number of infections and further pinpointed to 14 booths with both employee infections and contaminated environment samples.

One booth stood out, with all seven employees infected and five visitors infected.

Genome sequences of 110 samples found they were different from the virus in previous outbreaks in Harbin and Shulan in northern China and the outbreak in March in Beijing. Shared mutations showed the ancestral sequences were mainly identified in Europe, leading to the conclusion that the strain in the market was likely imported.

Investigators excluded other origins of infection, including contact with overseas, with thorough epidemiological investigations of each infected person.

Investigators examined salmon, the only imported commodity sold at the booth, and found six out of 3,582 samples outside the market were positive – five of them were from the same company that supplied the salmon to the booth.

Genome sequencing and genotypes of mutations found that the strains in the fish sample were a very similar virus strain to that in the market.

“Given the above-mentioned facts, we speculate that the Covid-19 resurgence in Beijing was likely to be initiated by an environment-to-human transmission originated from contaminated imported food via cold-chain logistics,” the authors wrote.

They warned of large outbreaks caused by such transmission and called for cold-chain imported food to be monitored.

“Even with low probability, such viral transmission would cause large-scale outbreaks if not being intervened immediately after the first cases. Regional guidelines on Covid-19 prevention and control should integrate surveillance of the cold-chain imported products, especially those from epidemic regions for the Covid-19,” they wrote.

The investigation were carried out by researchers from the Beijing Centre for Diseases Prevention and Control, the Institute of Medical Biology at the Chinese Academy of Medical Sciences and the Beijing Institute of Genomics. SOUTH CHINA MORNING POST