Did the virus that caused a worldwide pandemic make the jump to humans via frozen food? That was one hypothesis put forward on 9 February by a joint World Health Organization and Chinese investigation into the origins of SARS-CoV-2.
Frozen animals were found on sale at Huanan market in Wuhan, China, the place where the virus was initially detected, the team behind the work said. In a press conference, Peter Ben Embarek, the head of the investigation, said: “We know the virus can survive in conditions that are found in these cold, frozen environments, but we don’t really understand if the virus can transmit to humans.”
The idea that the coronavirus was carried inside or on the surface of frozen food, which has been advanced by Chinese state media, could place the source of the virus beyond China, from an animal imported from another country.
Yet it is far from clear whether the virus could survive in an infectious form via frozen food. “I would say it’s extremely, extremely unlikely the virus would have spread through that type of route,” says Lawrence Young at the University of Warwick, UK, who specialises in human virology.
The reason why, according to Young, is that SARS-CoV-2 is an enveloped virus, meaning it is covered with a fatty, lipid membrane that it uses to infect human cells. This membrane is very vulnerable to cycles of freezing and thawing, as can happen during the transit and sale of frozen food. Stripped of this envelope, such viruses cannot infect people.
A review by Jie Han and Zue Zhang at Xi’an Jiaotong University and their colleagues of evidence on spreading the coronavirus via food concluded that “major knowledge gaps exist” on the role that frozen food plays. “Data are lacking on the long-term survival of SARS-CoV-2 under freezing temperatures (-10°C to -20°C) that are frequently encountered on the storage and transport of frozen foods,” the team wrote.
Just one study, a preprint, has tried to obtain that data. Researchers put the virus into cubes of pork, chicken and salmon, finding no decline in the viral load after 21 days in a lab at refrigeration temperatures of 4°C or at a standard freezing temperature of -20°C. However, it isn’t clear whether the viral load was still infectious to humans, and the experimental parameters may not reflect real-world viral loads or conditions in supply chains.
SARS-CoV-2 may have a rough time when frozen food is transported. During air travel, temperatures drop to between -20°C and -30°C in cargo holds when planes are airborne, rising to a much higher temperature when they land.
By ship, the virus could suffer from the “salty air issue”, in which salt levels in the air can affect the number of viruses that can survive, says Julian Tang at the University of Leicester, UK. Changes in humidity on frozen food’s journey may also negatively affect SARS-CoV-2, as the lipid membrane can be disrupted by taking on fluid from the air.
We do know that the virus has been found to persist on the packaging of frozen food. After two workers at Qingdao Port in China tested positive in September 2020, SARS-CoV-2 was found on 50 of 421 samples of frozen cod packaging. The other possible route is if the virus is transported inside the frozen meat or fish itself. “If it’s part of the meat, it has more protection,” says Tang.
However, Rodney Rohde at Texas State University says that despite the virus being found on packaging, it doesn’t mean it is in a viable state that could infect our cells. “One must remember that any viral genetic material may be found on all types of surfaces, including frozen surfaces. But molecular PCR tests [that identify the presence of the virus] do not differentiate ‘viable’ from ‘non-viable’ virus.”
Even if the virus was still in an infectious form by the time it reached a person in Huanan market, there are still questions over how they were actually infected. Cooking the meat would kill the virus, as would the gastric acid in our stomachs, says Young. However, if the food was raw or not properly cooked, people could potentially be infected from surfaces during food preparation or via the upper respiratory tract while chewing the food.
“Overall, the probability [of infection via frozen food] is low. [But] if it happens once, a one in a million event, and it’s enough to seed the virus in the human population, you might get that spread,” says Tang.
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