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David Duffy

"this possibility is addressed in only one sentence" - the abstract of Worobey et al clearly states the actual argument: "these analyses provide dispositive evidence for the emergence of SARS-CoV-2 via the live wildlife trade and identify the Huanan market as the unambiguous epicenter of the COVID-19 pandemic."

They cannot, I think, exclude hypotheses where the CCDC labels samples where virus was present as coming from the southwestern corner of the market, where one author had seen live raccoon dogs being sold, or that two infected laboratory workers shop for live animals there. The paper has also to be read along with their other paper - Pekar et al, and the Gao et al paper that preceded them, which address the question of where the A and B lineages arose. Also see the N&V article

The Guardian article, at least, reads to me as well measured.


David, The Nature piece -- which I had not seen, thank you, -- quite rightly notes: "Some virologists say that the new evidence pointing to the Huanan market doesn’t rule out an alternative hypothesis. Namely, they say that the market could have just been the location of a massive amplifying event, in which an infected person spread the virus to many other people, rather than the place of the original spillover."
As far as I can tell nobody denies that the market is the most likely place that amplified the spread. And while that's super important to establish empirically, it does not speak to the politically salient question whether this originates in a lab leak. This is why I call the reporting irresponsible and misleading, and I stand by that for now.

David Duffy

Dear Eric - the distribution of sars-cov-2-positive environmental samples within the market (p15) is what drives the "dispositive evidence", with "five SARS-CoV-2-positive environmental samples [being] taken from a single stall known to be selling live mammals in late 2019...highly suggestive of infected animals having been present at the Huanan market at the beginning of the COVID-19 pandemic...the odds of detection [of a positive environmental sample] decreased by 2.8% per meter away from a live mammal vendor and by 6.6% per meter from a known human case (for N=6, Table S8), indicating that estimated effect on sample positivity due to proximity to live mammal vendors was farther-reaching than that of stalls with human cases".

The alternative hypothesis that the virus entered the market via a human carrier has to explain this particular intra-market pattern. Maybe everyone who shops there goes to see the animals, and the state of the cages etc helps the virus survive on surfaces. The dataset is not exactly ginormous, so I don't regard this as knock-down conclusive.


Dear David, look, first this is a classic case of underdetermination.

Second, the paper exhibits a tendency to treat confirmatory evidence as "dispositive," while treating lack of evidence as defeasible. Let me offer two examples: (I), as it notes when the Wuhan market was closed down "Some 457 samples from 188 individual animals corresponding to 18 mammals species were screened for active SARSr-CoVs infection via qRT-PCR from “within and outside Huanan Market”, and no positive SARS-CoV-2 samples were identified." (p13) It does not explain why this is a problem for the argument, or why we should set it aside. Rather, it goes on in the same paragraph to conflate this testing with a much broader (and less pertinent) testing program (which also did not provide positive evidence) and explains (quite plausibly) with it can be ignored. Again, as the Nature piece notes (and I am grateful for you to call my attention to it), "none of the studies contains definitive evidence about what type of animal might have harboured the virus before it spread to humans." That's the core of the evidential problem, and this is why it is so odd that on the authority of a stake-holder (the Chinse CDC--to which some of the authors have disclosed ties), the lab-leak is not even tested as a competing hypothesis or a null hypothesis (which with their spatial analysis they could do).

(II) The paper has a tendency to discount what it calls "putative index cases" unrelated to the Hunan market, (p.3) while acknowledging that all the cases (including the ones connected to Wuhan market) are unlikely to be the first one. I don't see how that avoids the perception of cherry-picking.

So, yes, I think this paper leaves no doubt that the market was the source of initial spread. I think it's plausible that there was a spillover event there. But as far as I can see only the Nature article reports the grounds for caution.

Lorna Salzman

In Viral (Chan and Ridley) and elsewhere there are firm statements indicating that despite tests of thousands of animals at the market, not one showed any signs of a virus. Has any contrary evidence appeared? Saying the virus spread from the market is not the same as saying that it ORIGINATED there. How can scientists do a study without stating clearly that a market
visitor (a lab worker or housewife or shopper) could have brought it there originally.? If I were editor of a peer reviewed journal, I would insist on this being included in the study before publishing it. Why didnt they?

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