But small, sporadic clusters have continued to resurface, despite some of the world’s strictest border restrictions and quarantine measures.
Apart from incoming travelers, Chinese officials suspect the virus is being brought by another culprit — imports of frozen food.
But China claims it has proven that it’s possible to contract Covid-19 from food packaging and is doubling down on efforts to prevent it.
Jin Dongyan, a virology professor at the University of Hong Kong, said while there is such a possibility, the CDC did not provide solid evidence of the transmission. He said the workers could have contracted the virus from elsewhere and then contaminated the food packaging. The missing step, he said, would be to compare the genetic sequences of the virus in the Qingdao workers and in people who handled the food at the source of the imports.
“Every virus has its own marks. If they match, then we can say there is a chain of evidence,” he said.
This week, Chinese authorities again tightened procedures, after a worker for a frozen food company in the port city of Tianjin tested positive for the virus last weekend.
On Monday, the Chinese government announced that all shipments of refrigerated and frozen food imports must be disinfected before they are allowed into the market. The requirement includes both the inner and outer packaging of these products, and the vehicles used to transport them.
“We will earnestly implement preventative disinfection of cold-chain food imports at ports of entry and strengthen inter-agency cooperation to stop the importation of Covid-19 through cold-chain food products,” Bi Kexin, a senior official in charge of food safety at the General Administration of Customs, said at a news conference Thursday.
The extraordinary measures are in addition to extensive screening that has already been introduced.
As of Thursday, Chinese customs said it had halted imports from 99 food companies across 20 countries where foreign factory workers were said to have contracted Covid-19.
Meanwhile, customs authorities across China have stepped up coronavirus testing on frozen food imports, especially seafood. As of Thursday, they had conducted random spot checks on some 873,000 samples, among which 13 were said to have tested positive.
Blaming the imports
China became suspicious of imported frozen food after an outbreak emerged from the largest wholesale food market in Beijing in June.
Wu did not elaborate on how he reached the conclusion, but other Chinese scientists and health officials have also pointed to imported frozen seafood as the likely culprit of that outbreak.
Jin, the virologist from the University of Hong Kong, remains skeptical. “This is a very weak paper. Nothing can be concluded from the study,” he said.
Is it possible to catch Covid-19 from food or packaging?
The coronavirus spreads mostly person to person through respiratory droplets when an infected person coughs, sneezes or talks. While it is technically possible to catch Covid-19 from food or packaging, experts say it requires the convergence of a series of low-probability events.
First, an infected worker would need to contaminate the food or its packaging with a virus load high enough to cause an infection — possibly by sneezing, coughing or shouting over it without a face mask.
Then, the virus must survive the long journey of international shipping and remain active on the surface while waiting to be unloaded and unpacked. From there, a food handler on the receiving end would have to touch the virus before touching their nose or mouth to become infected.
Previous studies have shown the viability of novel coronavirus differs from hours to weeks, depending on a number of factors, including the temperature, humidity and what kind of surface it is on.
“Zero tolerance” approach
Dale Fisher, an infectious disease specialist at the National University of Singapore, is studying how long the novel coronavirus can survive on refrigerated and frozen meat and salmon. His findings will be used to assess the potential of outbreaks emanating from imported food.
He said workers on the receiving end of the food shipments should practice good hygiene by keeping their work surfaces clean and washing their hands frequently. But he doesn’t believe consumers are at risk of catching the virus from frozen or refrigerated food packaging, because every time the product gets moved around or touched, the virus gets diluted.
“Because if it’s uncommon for those early food handlers, then further down the list, when it’s been brushed, wiped, moved around, put on one shelf and moved to another shelf… it would be too diluted when it gets to (the consumers),” he said.
In most countries, even if imported frozen food did infect a food handler, it wouldn’t be noticed due to the large amount of active cases, Fisher said. “You’ll only notice it in countries with no cases,” he said.
China, on the other hand, has doubled down on its scrutiny on overseas shipments. But the widespread screening China deploys might not be applicable to other countries where infections are rampant, Fisher said.
“Clearly, in the US and Europe, there’ll be absolutely no use in testing food, because there are so many cases spread by people,” he said. “But China has really zero tolerance towards Covid-19.”
“When there are trillions of tons of food moving around the world, even an unlikely event will happen a few times.”
But even if there is such a risk, there is no need to ban food imports, according to Fisher.
“The intervention is just to make sure the source of the food has got Covid-safe measures so the food can’t be contaminated,” he said. “We’re not saying stop sending food around the world.”