The announcement by Chinese officials was as striking as it was alarming.

After spending weeks investigating a mysterious coronavirus outbreak in the port city of Qingdao, officials from China’s Center of Disease Control declared confidently on Oct. 17 that they had found the culprit: packages of frozen food imported from abroad.

The researchers said they found and isolated live samples of the virus on the outer packaging of cod shipped into China, a country that otherwise had gotten rid of the virus domestically. Two dockworkers who handled the packages were apparently the first to be infected, and they spread the virus at a hospital in Qingdao.

It was proof, according to China CDC researcher Wu Zunyou, that the virus could survive long trips in a deep freeze and still infect people. “Imports from abroad caused the spread and transmission of this outbreak,” Wu told reporters in Beijing.

But in the weeks since, the Chinese findings have set off a flurry of heated discussion among international researchers about the likelihood of the coronavirus spreading across borders on cold-chain food products, highlighting key questions about the ways the virus can be transmitted — and how international authorities and governments should perceive these risks.

In the United States, as the pandemic rages, an increasingly pressing worry has been airborne transmission — which appears to be the key to large super-spreading events. Meanwhile, transmission from surfaces has been played down by experts, who have emphasized that this route is not thought to be a common way the virus spreads.

But in China, where cases are increasingly rare and the government has adopted a no-tolerance policy for new infections, a growing emphasis has been placed on identifying less likely sources of infection.

The new claims from China have presented a dilemma for international bodies such as the World Health Organization and deeply divided experts. Some say the Chinese data, which include genetic sequences of the virus from the packaging that matched viral strains in Europe, appear to be persuasive. Others dismiss the evidence as inconclusive.

The evidence that the fish was the source of the infection “are very weak,” said one U.S. CDC scientist who spoke on the condition of anonymity because they were not authorized to talk to media outlets. “This is typical of the difficulties inferring directionality. Maybe the workers contaminated the fish. Maybe another worker who is infected in the same complex infected both the workers and touched the fish."

Still others say that even if frozen food packaging were a transmission vector, agencies such as the WHO should not overstate its likelihood, because it could cause unnecessary panic or disrupt the international food trade.

“The food industry is naturally enormous, and you certainly don’t want to disrupt food chains or cause consumers to lose confidence for a very minor route of transmission,” said Dale Fisher, a professor at the National University of Singapore who chairs the WHO’s outbreak response network.

In recent months, there has been sufficient circumstantial evidence from several outbreaks, including in Qingdao, Beijing, New Zealand and Vietnam, combined with the recent Chinese results, to show that frozen food can be a transmission vector, said Fisher, whose lab is researching how long the coronavirus can remain infectious under cold temperatures.

Authorities have been understandably reluctant to draw attention to the risk, but perhaps they should, Fisher said, adding: "I don’t understand the reasoning for denying the possibility.”

For the Qingdao case, meanwhile, the China CDC also cited the genetics of the virus itself to make its case.

When Chinese authorities uploaded 11 viral sequences from Qingdao last month to a global database housed by the GISAID Initiative — two from the infected dockworkers, the rest from the frozen packaging — the initiative noted that the sequences have a unique identity suggesting that the cases are all related.

Specifically, the viruses are from a genetic grouping, or clade, “not recently seen in China, but frequently in Europe and other countries, indicating likely import," GISAID said.

Furthermore, all of the viruses have a telltale deletion of the same three genetic bases, and this deletion has not been detected before in coronavirus genetic sequences currently in the GISAID database. Therefore, it appears very clear that the viruses on the packaging, and those that infected the workers, are linked.

Yet the genetic connection alone does not itself prove that the infection came from the packaging, several experts noted. For instance, what if the dockworkers became infected some other way and then coughed on the packaging themselves? It is not entirely clear how Chinese authorities, who did not respond to The Washington Post’s requests for comment, can rule this out.

“Is it theoretically possible that workers got sick from packaging? Yes. Do we have evidence that this is a common mode of transmission? No," said Donald Schaffner, an expert on food sciences and viral transmission at Rutgers University.

Officials from the WHO, which previously played down the frozen packaging theory, acknowledged China’s claims in a statement to The Post. “It is important to stress that these cases are relatively few,” the agency said.

“We are aware of previous reports from countries including China, Germany, [the] Netherlands and New Zealand where virus was found on packaging of frozen food through PCR testing, but this is the first time that the live virus was isolated."

One possibility for what’s happening is that China has controlled the virus so well that it is now able to detect even the most unlikely types of transmission — by ruling everything else out. In Qingdao, for instance, millions of people were tested in a matter of days once the outbreak was detected, which presumably gave Chinese authorities more confidence about the original source of the infection.

Qingdao isn’t the first case where Chinese experts have blamed frozen packaging for reintroducing the virus. On Sunday, authorities in Tianjin said they would test all cold storage sites and put the port city on “wartime mode” after a storage worker tested positive after handling frozen pork from Germany. Officials said they also found the virus on fish from India.

In Beijing over the summer, Chinese experts also pointed to frozen fish as the source of an outbreak, and have recently published their findings in detail, claiming that epidemiological and genetic evidence support their case.

In June, remarks by Chinese CDC officials that packaging on imported salmon may have been the vector for an outbreak at the Xinfadi seafood market led Chinese buyers to cancel salmon orders from Norway. In a new paper, researchers affiliated with the Beijing CDC said they sequenced 110 samples from the seafood stalls and from infected patients and found that they formed a genetic cluster and appeared related to “ancestral” sequences found in Europe.

“Our finding is particularly important for countries where community transmissions are contained or suppressed,” the researchers wrote. “The virus could be reintroduced via cold-chain transportation of contaminated items and might initiate an outbreak.”

Jin Dong-yan, a virologist and associate dean at the University of Hong Kong, said it’s well-established that viruses survive longer in freezing temperatures.

“The speculation is that someone breathes or talks and droplets land on the surface of the packaging, and under low temperatures the virus survives longer and is still infectious after several days,” Jin said. “But that doesn’t change the fact that it’s still extremely rare for people to contract the virus through food packaging."

Complicating matters further is that in China, there’s a political bent to the claims of virus importation. Nationalist-leaning government officials and state media have advanced the dubious theory that the coronavirus pandemic didn’t start in China’s Wuhan at all, or that it originated in Europe or the United States but only exploded once it reached Wuhan in December.

That theory is generally dismissed by experts, including many in China and Hong Kong. But some Chinese officials speculated that the new research on cold-chain shipments could strengthen the hypothesis. Wu, the China CDC epidemiologist, told the nationalist Global Times newspaper that outbreaks during the summer in Beijing, Dalian and Qingdao occurred after China had largely contained the virus domestically, were all caused by seafood, and “reminded us of the initial outbreak in Wuhan.”

“Was that also caused by imported seafood?” Wu was quoted as saying Thursday. Wu declined to comment further when contacted by The Post.

Whatever the risk, China is already taking precautions. Last month, China’s cabinet announced new guidelines requiring thorough disinfection of cold-chain food packages from its list of “high-risk” countries, which include Brazil, Italy and the United States. China’s customs agency announced Oct. 28 that it found traces of the virus on frozen packages of seafood from Russia and the Netherlands and would limit imports from the companies and fisheries involved.

Still, the Chinese CDC itself has noted that the risk of this type of infection remains low for package handlers and practically nonexistent for consumers, adding that the main route of transmission was still respiratory droplets and close human contact.

As of Sept. 15, various Chinese provinces had collected nearly 3 million samples, including 670,000 swabs from packages and the rest from package handlers and surfaces in shipping centers, but results showed the coronavirus genetic material on only 22 samples, the agency said.

In the end, the verdict seems to be that the route of transmission that China is highlighting is possible, but rare — whatever the merits of the cases that have been highlighted.

“Surfaces can occasionally be a source of transmission," said Dave O’Connor, an expert on the genome of the virus at the University of Wisconsin at Madison. "They do not appear to be a major, or the major, source of transmission in areas where the virus is already endemic. If you have otherwise eradicated the virus, such as New Zealand or this region of China, vigilance will be required to prevent reintroductions by both goods and travelers.”

Lena H. Sun contributed to this report.