Even closer to the SARS-CoV-2 case, the 1977 flu pandemic killed ~700k people and near-certainly arose from a research accident, probably a failed vaccine trial:

> The 1977-1978 influenza epidemic was probably not a natural event, as the genetic sequence of the virus was nearly identical to the sequences of decades-old strains.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4542197/

That article argues that vaccine trial accident doesn't count as a "lab accident", which seems like legalistic wordplay to me; but I believe the historical context is good.

But in 1978, the WHO wrote:

> Laboratory contamination can be excluded because the laboratories concerned either had never kept H1N1 virus or had not worked with it for a long time.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2395678/pdf/bul...

It took decades for the consensus to change, and there's still been no official admission (though note the unverified personal communication from C. M. Chu via Peter Palese in Gronvall's paper, 27 years after the accident).

The USSR successfully manipulated many prominent Western civilian scientists (and subsequently Western newspapers) into buying the "bad meat" Soviet cover-up story, hook, line, and sinker: to the point that they publicly chided the American military for still believing in the "anthrax factory leak" theory:

> But Dr. Alexander Langmuir, a former director of epidemiology at the United States Centers for Disease Control, who presided at the session, said Tuesday that based on what he knew so far, "the current position of the U.S. military needs thorough re- examination; that is clear.'

> Dr. Philip Brachman of Emory University, a U.S. anthrax expert who has advised the Central Intelligence Agency on the incident, said the Soviet talk was a "landmark report" providing "a pretty good indication that this incident was an outbreak of gastrointestinal anthrax" from eating contaminated meat.

> Dr. Alexander Langmuir, a former chief epidemiologist for the federal Centers for Disease Control in Atlanta, agreed that the Soviet account was "very credible so far" but added that he hoped to obtain further details during the Soviets' weeklong stay.

The "bad meat" Soviet coverup story was fawningly parroted in _Science_ in 1988: https://www.science.org/doi/10.1126/science.3358121 (full text at https://sci-hub.se/10.1126/science.3358121) and also ended up in prominent American newspapers like the New York Times (https://www.nytimes.com/1988/04/14/world/russians-explain-79...) and Wall Street Journal (https://www.washingtonpost.com/archive/politics/1988/04/13/s...), from which the above quotes are taken.

How did this happen? Why did this work so well? Because scientific methods designed for operation in adversary-free environments are catastrophically unfit for drawing dependable conclusions when intelligent motivated adversaries have tampered with the available data. The Western civilian scientists were thinking like...scientists, so they got easily manipulated into producing and disseminating literal Soviet disinformation while believing they were furthering the causes of science and of fighting anthrax. Oops:

> Three Soviet officials came to visit the National Academy of Sciences in Washington, D.C., on 11 April: Pyotr Burgasov, retired deputy minister of health; Vladimir Nikiforov, infectious diseases chief at the Moscow Institute for the Advanced Training of Physicians; and Vladimir Sergiyev, director of the Institute of Parasitology and Tropical Medicine. They gave the same explanation as in 1980, but provided many more details, convincing some long-time doubters that the account was true. U.S. intelligence officials still maintain a military facility was involved.

Spoiler alert: Pyotr Burgasov was part of the response to the leak and part of the Soviet biowarfare program.

The argument for a natural origin of Sverdlovsk anthrax is tragically familiar: establishing a highly plausible way it could have been zoonosis, along with providing evidence consistent with a zoonosis. From the _Science_ article:

> The citizens of Sverdlovsk did not have to look so far to find the bacillus. It has been endemic to the region for centuries.

> The source of the outbreak was traced to a single 29-ton lot of bone meal (cattle feed) sold in March from a factory in Aramil, 15 kilometers to the southeast of Sverdlovsk. It must have taken in and ground up the bones of animals who died the previous year of anthrax. Its product clearly contained live anthrax. An official investigation found that the factory did not follow the prescribed heating and pressure treatment methods. The feed it produced went to a state farm, where veterinarians inoculate the animals, and to private owners, whom state veterinarians often do not bother to visit, according to Burgasov. On the approach of the May Day holiday, animals are slaughtered, generally the weakest (in this case, the sickest) first. The meat from the private butchers is thought to have triggered the outbreak, Burgasov said.

It's only collapse of the USSR that enabled the truth to come out in public. Two Soviet pathologists, against KGB orders, had kept some autopsy samples, which (along with a defector's testimony) annihilated the "bad meat" cover-up story. Turns out the outbreak of airbone anthrax that happened in the same city as the industrial-scale weaponized anthrax spore factory was indeed caused by the industrial-scale weaponized anthrax spore factory! https://www.theatlantic.com/health/archive/2016/11/sverdlovs...