When a cluster ofappeared in Qingdao in October 2020, local health authorities scrambled to test 11 million residents of the Chinese seaport in just five days. The extensive tracing efforts led them back to two dock workers who had been infected with COVID-19 in late September.
It was never confirmed how the workers became infected, but the Chinese Centers for Disease Control revealed it was able to detect genetic traces of SARS-CoV-2, the virus that causes COVID-19, on imported frozen cod packages at the docks. It did not state where the imports had been shipped from, but the agency announced in October that this "proved" contact with contaminated packaging could lead to COVID-19.
It was a suggestion at odds with the rest of the world. The US Food and Drug Administration, Australia and New Zealand's Food Standards board and Europe's Food Safety Authority all concluded there is little to no evidence showing SARS-CoV-2 can infect individuals via food packaging. But on Feb. 9, at a press conference detailing findings from a joint WHO and China investigation in Wuhan, the frozen food theory became embroiled in the most controversial and politically loaded question of the pandemic: Where did the coronavirus come from?
Over the past year, two parallel theories have emerged to explain COVID-19's appearance in Wuhan in December 2019. One posits the virus arose naturally and jumped from a bat, possibly through an intermediate species, into a human. The other suggests it may have accidentally leaked from a lab in the city and insidiously spread through the population.
The new hypothesis was given further weight this month. WHO and Chinese investigators who visited Wuhan wet markets and laboratories on a fact-finding mission in February suggest the former theory is the most likely, dismissing a lab leak as "extremely unlikely." But they also offered up an alternative theory for the original outbreak: frozen food or animals, imported into Wuhan and sold at wet markets, sparked a cluster that ballooned into a pandemic.
Peter Ben Embarek, food safety expert and leader of the WHO team investigating the origins of COVID-19, said at the Feb. 9 press conference that "it would be interesting to explore" whether infected, frozen wild animals could have introduced the virus or viruses into market environments. However, he says, a lot of work needs to be done to better understand these pathways.
In offering this alternative, the WHO's investigative team tacitly endorsed a contentious hypothesis that had been gaining momentum inside China for months. State-run media outlets first reported small outbreaks sparked by food and food packaging in July 2020, at times forcing frozen salmon off the shelves. In August, China said the virus was found on shrimp packaging from Ecuador, though no infections resulted from the contamination. But it was the statement released by China's CDC in relation to the Qingdao dock workers that saw the theory really take hold.
The implication, championed by prominent Chinese scientists, is that the pandemic may have begun outside of China. Tensions have flared between Beijing and countries like the US, Australia and India over China's handling of the pandemic.
On currently available evidence, the frozen food theory seems more preposterous than a lab leak and requires a much more convoluted pathway from animals to humans. It may be another way to help deflect blame away from China's initial handling of the pandemic. At this intersection of science and politics, throwing the frozen food hypothesis in the mix has further complicated the already messy hunt for the origins of the disease.
Researchers have demonstrated that SARS-CoV-2 can survive conditions as low as minus 20 degrees Celsius and that the virus thrives in damp, cold conditions. Because the coronavirus can linger on a cold surface, there's a chance humans may become infected by handling products contaminated with viral particles. There's been a moderate incidence of COVID-19 outbreaks in meat packaging facilities in the US, the UK, Germany, Australia and a handful of other nations.
Publicly available research on frozen food as a source of outbreaks is scarce. The WHO's database of COVID-19 research lists just 26 results when searching for "frozen food" and only eight discuss the cold chain and food packaging as SARS-CoV-2 transmission routes specifically.
China has been screening imported goods for the coronavirus as a precaution since June, after it detected genetic fragments of SARS-CoV-2 on arriving food. Over 1.4 million samples have been analyzed since. Yet, according to one report published in China's CDC Weekly journal on Jan. 8, only four cases of cold chain contamination were identified in the country in 2020.
As of Nov. 30, 2020, viral RNA was detected in less than 0.05% of sampled products -- a staggeringly low percentage. Importantly, this viral RNA doesn't prove an infectious virus is present. It simply means genetic material has been detected.
China has been able to test imports stringently because the country has controlled its outbreak. Around the world, where the pandemic has spread chaotically, it's a different story. "In many other countries, they do not test so strictly," says Chenyu Sun, a physician at AMITA Health Saint Joseph Hospital in Chicago. Sun recently published a letter to the editor in the journal Public Health suggesting cold chain transmission "cannot be ruled out."
No cases of contamination have been reported outside of China but, as Sun notes, resources are focused elsewhere in places where coronavirus is out of control. Without testing, it's impossible to know if there's contamination of imported food across the globe.
Even if contamination of products is occurring, there's no credible evidence to show people are being infected via the cold chain. In the case of the Qingdao dock workers, live virus was found in the packaging (at low levels), but it couldn't be isolated from food.
Previous research shows frozen food isn't particularly conducive to spreading a coronavirus. An epidemiological investigation during the 2003 SARS epidemic looked at antibodies to the SARS virus in wild animal traders in the city of Guangzhou. Almost 60% of traders exposed to wild animals had antibodies to this coronavirus but that figure dropped to just 10% for those trading frozen food. SARS-CoV-2 is more transmissible than the original SARS virus, but this provides at least a foundational assessment of how uncommon coronaviruses on frozen food elicit infections.
Even WHO food safety expert Ben Embarek, talking to Science magazine, suggests "it is probably an extremely rare event" for the coronavirus to be found on or in frozen food. 'We have to separate the situation in 2020 with imported goods in China, and the situation in 2019, where that was not a possible route of introduction."
Despite this seeming admission that it's not possible, he also tells Science "it's potentially possible, so it's worth exploring."
But why investigate frozen food as an origin of the pandemic when it appears so unlikely? That question is linked to the Huanan Seafood Market.
Into the unknown
Many of the initial cases detected in Wuhan in December 2019 were clustered around the Huanan Seafood Market, a bustling wholesale market selling wildlife products, live animals and frozen goods. As epidemiologists investigated COVID-19 cases in late 2019, the market became a key piece of the origins puzzle. On Jan. 1, 2020, it was shut down.
Sampling of the market on Jan. 1 and Jan. 12 found SARS-CoV-2 in 33 of 585 environmental samples, detecting the virus on door handles, stalls and sewage water. The coronavirus was definitely there -- but how did it get there?
There are two lines of thinking.
The first: An infected human carried it into the market, where it was then able to spread from person to person. This fits the data. There were over 1,000 tenants at the Huanan Seafood Market, and the coronavirus thrives in crowded locations. It fits with the epidemiological data, too -- patients in Wuhan who had not visited the market were infected in early December. The market was more likely an amplifier of COVID-19 spread than the beginning.
The second: Someone -- a trader, a patron, a seller -- was infected inside the market, leading to the first outbreak. This is where the frozen food theory comes into play. It suggests the virus may have hitched a ride on some of this wildlife outside of Wuhan and made its way to the market. Could that be true?
Part of the WHO's investigation was to venture to the market and hunt for clues. In an interview with The New York Times, Peter Daszak, a member of the WHO team, said wildlife was being brought into Chinese markets from regions in southern China and Southeast Asia, where relatives of the coronavirus circulate.
For SARS-CoV-2 to be brought in on frozen food, it had to be circulating somewhere else in the world, like the regions Daszak suggests, prior to December 2019. And if that's the case, there should be other cases of COVID-19 in those areas. But we've not seen any. "The genetics of the virus prove that there was only a single source in November of all human COVID," says Nikolai Petrovsky, a vaccine developer and professor of endocrinology at Flinders University.
More than a year after the pandemic began, we've no evidence of outbreaks earlier than those in Wuhan in December 2019. In addition, animal samples obtained from the market, Daszak says, tested negative for SARS-CoV-2. Ben Embarek suggests checking suppliers and farms to test animals and their environment for signs of the coronavirus -- but why has this not already been done?
The lack of evidence for the theory has some scientists believing a continued focus on frozen food may derail investigations or could further obfuscate the truth of the virus's origins. "This could quite simply be the biggest smokescreen in history," Petrovsky notes.
The next right thing
Still, all hypotheses regarding the origin of the coronavirus remain on the table, according to Tedros Adhanom Ghebreyesus, director-general of the WHO. That includes the controversial lab leak hypothesis that initially appeared to be dismissed by the investigations team on Feb. 9.
When asked during the press briefing to put a figure on the likelihood of each theory, team member Marion Koopmans, a Dutch virologist, said "going into exact percentages is really overstating what can be done." Instead, the researchers use a five-phrase classification to help organize and plan future studies, listing from "extremely likely" to "very likely."
Of their theories, the most likely origin of SARS-CoV-2 continues to be a "spillover" event, where the virus jumped from a bat to humans. Bats and people don't have a lot of close contact, so the running hypothesis is this leap was made via an unknown intermediate species (but almost certainly not a pangolin, as was once suggested).
Where does the frozen food theory sit? That's not clear. The publicly available scientific evidence would suggest it should be classified, by the WHO's metrics, as "extremely unlikely." CNET asked Ben Embarek and other members of the WHO team, including Koopmans, Dominic Dwyer and Thea Fischer, if the team had acquired more information to support the theory, but did not receive a reply.
The WHO has been criticized for acting too slowly on information from China in the initial stages of the outbreak, while simultaneously praising the country. The organization relies on its member states, which include China, for funding, but it has no power to bind or sanction them. The lack of independence has been most telling in how the organization's messages have aligned closely with those out of China since the beginning of the crisis.
This same concern over independence has permeated the debate around the Wuhan origins investigation. The team has had to navigate a tense political environment and walk a fine line between scientific inquiry and forensic investigation.
Since the team's investigation ended, multiple reports have suggested there were "heated debates" between scientists from the WHO and China over access to critical data. A report by The Wall Street Journal alleges Chinese scientists didn't allow the WHO team access to data from early cases, instead providing summarized information to investigators. This has only served to further question Beijing's transparency in the early days.
We should know more soon. The WHO plans to release a summary report of the findings from Wuhan this week, and a full report is expected to come later this year. With it, the frozen food theory will be put under the microscope. Will it be able to stand up to scrutiny? The data is limited, but the early evidence suggests not.