Recent attempts to blame China for SARS-CoV-2 have argued that it could have been released from a poorly regulated laboratory in Wuhan, as @IgnatiusPost does here. This sounds more reasonable than the bioweapon accusations, but there is strong evidence against both claims. 1/x https://twitter.com/IgnatiusPost/status/1245864149139066880

This 3/17 Nature Medicine article investigates. There are several reasons to disbelieve the "laboratory accident" claim. Most important: "the genetic data irrefutably show that SARS-CoV-2 is not derived from any previously used virus backbone." 2/x https://www.nature.com/articles/s41591-020-0820-9

This effectively rules out the bioweapon hypothesis; SARS-CoV-2 evolved on its own. Moreover, it did so very recently: "Estimates of the timing of the most recent common ancestor of SARS-CoV-2...point to emergence of the virus in late November 2019 to early December 2019." 3/x

That leaves two possible "laboratory accident" hypotheses. First, that a lab identified SARS-CoV-2 in the wild and gathered it for study, after which it escaped. Second, that SARS-CoV-2 evolved from another virus in laboratory cultures, then escaped. Both are very unlikely. 4/x

The "identification" hypothesis is unlikely because of timing. In order for it to be true, the lab would have had to isolate SARS-CoV-2 from among thousands (millions?) of other viruses, culture it, then accidentally release it, all in the span of weeks, at most. 5/x

The "in-lab evolution" argument is theoretically possible, as the Nature researchers note. But they also note that finding similar coronaviruses in pangolins with "near identical" structural features means SARS-CoV-2 almost certainly evolved in the wild. 6/x

They also note "a hypothetical generation of SARS-CoV-2 by cell culture...would have required prior isolation of a progenitor virus with very high genetic similarity, which has not been described." In other words, some intermediate steps needed to get to cultures are missing. 7/x

In summary, the authors conclude that because we can see "all notable SARS-CoV-2 features..in related coronaviruses in nature, we do not believe that any type of laboratory-based scenario is plausible." 8/x

There are urgent, legitimate questions about the COVID-19 pandemic's emergence, including why Beijing withheld genetic data and downplayed the outbreak's severity for weeks. But available evidence indicates it almost certainly did not emerge due to a laboratory accident. 9/x

To be sure, this is not the same as saying China has no *responsibility* for the outbreak; that is a larger question. But if we're asking whether Chinese government negligence at a lab was the cause of the outbreak, the answer is almost certainly no. 10/x https://twitter.com/ErikVoeten/status/1246493094654234627?s=20

More broadly, though, insinuating that the virus escaped from a lab in China by saying "well, there's no evidence that it *didn't*" is not only untrue, it amounts to disinformation that could further ratchet up US-China tensions and distract from more urgent priorities. 11/x