China’s health authorities have repeatedly said they are finding the Covid-19 virus on frozen food imports and have linked infections in the country to pig heads and seafood.
While some researchers and health authorities have raised doubts about frozen food as a virus transmission route, Beijing has suspended imports of products and introduced checks, tests and disinfection of packaging and containers, creating delays that clog ports and irk trading partners.
Several top Chinese scientists have further suggested that the Sars-CoV-2 virus that causes the Covid-19 disease may have arrived in the Huanan Seafood Wholesale Market in Wuhan city, the location of the world‘s first known outbreak, via frozen food imports, or what’s referred to as cold-chain transmission.
That theory featured prominently in a press conference in Wuhan on Tuesday, where an international team of WHO specialists presented the findings of a month-long probe into the origins of the virus alongside Chinese officials.
“Sars-CoV-2 can persist in conditions found in frozen food, packaging and cold-chain products,” said Liang Wannian, the National Health Commission official who led the Chinese side of the mission.
Liang also suggested the virus might have caused infections overseas before the outbreak in Wuhan but that they were not identified.
Beijing has repeatedly stressed throughout the past year that just because the virus was first detected in Wuhan it may not be where it emerged.
Daniel Lucey, an infectious disease specialist at Georgetown University Medical Centre in the US, said the frozen food route did not seem like a “very likely legitimate explanation” and would warrant comparative studies.
“Why Wuhan first? Of all the seafood markets in China and Asia and around the world how did the cold-chain packaging end up causing an outbreak in Wuhan first?” he said.
Dale Fisher, an infectious diseases doctor at the National University of Singapore (NUS) said it was “reasonable” for the WHO team to consider the theory.
“[However,] the cold-chain [transmission] theory really comes on the idea that there’s outbreaks happening in the meat processing plants in another country. But it’s very unlikely there was widespread disease spread happening before Wuhan,” Fisher said.
The WHO team in Wuhan said another more commonly held hypothesis that the virus was introduced to humans via an intermediary animal was most likely, but further research into the potential role of cold-chain products was needed
Another theory that the virus leaked from the Wuhan Institute of Virology, which conducts research into coronaviruses, was “extremely unlikely”, the team said.
But there may be a connection between the theories, said Peter Ben Embarek, the WHO team leader and a food safety scientist, at the Tuesday briefing.
It “would be interesting to explore” if an infected, frozen wild animal could have brought the virus into the Wuhan market, he said, adding that it remained unknown whether humans could become infected from a virus that had survived freezing conditions. The Wuhan market was known to sell wild animals, both live and butchered.
Chinese authorities began suspecting that cold-chain products could be responsible for the spread of Covid-19 following an outbreak at a Beijing wholesale food market in June.
The capital had gone 55 days without a new infection until an outbreak of more than 300 cases were linked to the market.
Investigators were initially flummoxed but then reported discovering genetic similarities between the outbreak and viral remnants found on salmon imported by a company that sold to a booth in the market linked with early infections.
It was “unclear” if the virus on the salmon was enough to make people sick, but it showed that the risk existed, the investigators, including those from Beijing Centre for Diseases Prevention and Control (CDC), wrote in the Chinese journal National Science Review in October.
Following the Beijing outbreak, Chinese authorities linked several infections in the country to imported food.
In November in Tianjin, a port city in northeast China, officials concluded that pig heads from North America had infected two workers, after finding genetically similar virus on the ground where the heads had accidentally been dropped. The Tianjin CDC did not say whether the heads themselves tested positive for the virus.
Cod in another port, Qingdao, where researchers isolated “live” virus from packaging, were also among products analysed and pinpointed as causing outbreaks.
Virus researchers elsewhere have said it is possible workers could become infected from handling a package that was sneezed or coughed on by a sick person in another country, but the chances are very low.
“All the stars would need to align,” said food virologist Lee-Ann Jaykus, a William Neal Reynolds distinguished professor at North Carolina State University.
China’s own statistics show this: only 0.36 of every 10,000 samples taken from food or its packaging tested positive for the virus by last month. And “positive” does not necessarily mean it is enough to infect someone.
Fisher of NUS said Covid-19 could survive at refrigerated, or cold-chain, temperatures during the transport of food and may be responsible for driving a rare infection given how much food is transported around the world.
But the evidence of infection via this route was “circumstantial”, he said.
Leo Poon Lit-man, head of the division of laboratory sciences at the University of Hong Kong’s School of Public Health, agreed that there was a lack of evidence. “It’s very hard to assess the risk based on only a few incidents,” he said, adding that he did not want to exclude any possibility completely.
Even so, some 124 cold-chain food producers in 21 countries have suspended exports to China, voluntarily or otherwise, China’s General Administration of Customs said last month.
China has temporarily barred companies with infected employees or virus-positive goods, and trade partners have complained about China’s rules, including at the World Trade Organization.
“China’s most recent Covid-19 restrictions on imported food products are not based on science and threaten to disrupt trade,” the US Department of Agriculture said in November.
On Tuesday, the US said it would not accept the WHO findings in Wuhan without independent verification. China has not provided the “requisite transparency” said State Department spokesman Ned Price.
Other products – such as Brazilian chicken wings, Saudi Arabian shrimp, Chilean cherries and Ukrainian milk powder – have been in the spotlight after reports that the product or its packaging tested positive.
These positive tests could merely mean there are very small amounts of the Sars-CoV-2 virus present that are not a risk to anyone, according to David Murdoch, epidemiologist at the University of Otago in New Zealand.
Meanwhile, it is accepted that the main way Covid-19 spreads is via close contact with sick people who are releasing virus particles into the air when they speak, cough or sneeze.
The relative risk of getting sick from touching a contaminated surface is thought to be “much, much lower”, according to food virologist Jaykus at North Carolina State University.
She said that left open the possibility the people linked to so-called cold-chain infections had picked up the virus elsewhere. “I haven’t seen anything compelling in the literature that rules out other explanations that probably are more likely.”
The WHO guideline for consumers says there is “currently no evidence that people can catch Covid-19 from food or food packaging”, but recommends that food handlers take measures to protect themselves.
“Some studies conducted on Covid-19 surface transmission show that the virus could survive a certain amount of time under cold storage conditions,” a WHO spokesman said in a written response to the South China Morning Post.
China has reported no cases of consumers becoming infected from contaminated food or packaging, China National Centre for Food Safety Risk Assessment deputy director Li Ning said last month.
But China seems to be alone in linking infections to virus found on food packaging.
By the end of last year, Hong Kong had tested 1,300 related frozen food samples and 2,200 cold storage workers. None tested positive, according to a recent government report.
Taiwan conducted a spot check last year of 44 imported items from four countries hard-hit by Covid-19 and found no traces of the virus.
In New Zealand, a warehouse worker at a frozen food logistics company became the first local Covid-19 case in the country after more than 100 days, but a government investigation ruled out cold-chain transmission.
A group of Chinese CDC researchers, including director Gao Fu, said they had found the most persuasive evidence of this kind of transmission after investigating cases in the eastern coastal city of Qingdao.
The team said they had isolated, for the first time, a “live” virus, or one able to replicate in cells, from packages of cod. Two infected workers had unloaded shipments of the fish days earlier, which researchers concluded was the likely source.
Microbiologist Emanuel Goldman of the Rutgers-New Jersey Medical School said the virus isolation was “proof of principle” that this kind of transmission could happen, but it would still be “very rare”.
However, Poon of HKU said it was not clear if the virus had come from fish packaging first or had been contaminated by a cough or sneeze from a sick worker. “It’s like the chicken and egg question, which came first?” he said.
In his comments on Tuesday, WHO team leader Ben Embarek repeated that for a virus that had survived on frozen goods: “We don’t really understand if the virus can then transmit to humans. A lot of work needs to be done.”