[jpeloquin]

Sure. P(lab leak origin | last bullet) = P(last bullet | lab leak) * P(lab leak) / P(last bullet). There isn't much evidence to constrain each term, and your post doesn't make any attempt to estimate them, but let's make some stuff up. The values change a lot if you take P(lab leak) to be any lab leak, or specifically a modified RaTG13 from WIV.

Let's go with the more specific scenario. Then P(last bullet | WIV lab leak scenario) becomes P(outbreak | WIV lab leak &c.) because we've conditioned on everything but the outbreak part. I'll guess this is 0.001, which is generous because it's very unlikely for a mere exposure to turn into a pandemic. Exposures to natural potentially pandemic viruses happen every day, most don't become a pandemic. P(WIV lab leak scenario) is difficult. There are a lot of different lab leak scenarios that could have happened instead, but we also don't care that much about trivial differences. Let's be really generous and say P(WIV lab leak scenario) = 0.25 because it's one of I think 4 plausible covid origin sites in Wuhan. Probably it should be lower because it's plausible that Wuhan was only the site of the first superspreader event, not the origin. And let's say P(outbreak) = 0.2 because we seem to get a potential pandemic every 5 years or so. Then P(WIV origin | outbreak) = 0.00125.

There are all sorts of problems with my analysis of course. Even if I haven't made any mistakes, the numbers I conjured up are entirely unconvincing. With only one datum, we have hardly any objective support for our probability estimates, and Bayesian analysis only gives a semblance of objectivity to our prior opinions.

Edit: I have no desire to support either side here, just to point out that probabilistic reasoning doesn't help much here. Whatever happened, happened, regardless of how (un)likely it was.

[voldacar]

I had a longer response but my browser crashed so I'll just try to summarize my thoughts. Our disagreement basically comes down to how we assign priors. I'm not really sure how productive a debate on priors can be because it's not like there are a bunch of previous similar scenarios that we can study in detail.

On P(WIV leak):

Wikipedia[0] lists 50 lab leaks (or more precisely, failure of lab biosecurity measures) in the past century. The actual number is probably much higher because people and institutions like to hide their mistakes. So the base probability of a lab leak happening from a lab that studies pathogens is only moderately low to begin with.

But P(WIV leak) is probably significantly higher than average here, because WIV had worse than average safety procedure.

- According to leaked cables from 2018, american officials & scientists were already concerned about unsafe practices & lack of properly trained personnel at WIV[1]

- WIV researchers collected unknown viruses from bat caves with only hazmat suits + surgical mask as PPE, and handled vials of viral vector specimens without even masks[2]. If they demonstrated poor attention to safety outside the lab, it is likely that they disregarded it within the lab as well

- I recall reading something similar involving Shi Zhengli but I can't find the link now

- Shi Zhengli admitted (!) that bat coronavirus research was performed under BSL-2 and BSL-3 conditions, both of which are significantly more lax than BSL-4[3]

On P(last bullet):

It's a pretty specific scenario, so this probability is very low. It's specific in a way that's meaningful and relevant though, it's not like the specificity came from extraneous things like "and the virus' genome is between 29 and 30 kb" or "and the outbreak occured in a city whose name starts with W".

On P(last bullet | WIV leak):

This probability is essentially a function of the infectiousness of the viruses in the WIV.

> it's very unlikely for a mere exposure to turn into a pandemic. Exposures to natural potentially pandemic viruses happen every day, most don't become a pandemic.

Strongly disagree. The viruses in WIV were deliberately altered, via either natural selection in host cells or direct genetic manipulation, to be significantly more infectious in human cells than a random virus found in the wild would be. It is unlikely for human exposure to a random new virus to lead to a pandemic. That is not the case for human exposure to a new virus that has been altered to be much more competent at infecting humans. Especially considering that these viruses we're talking about are coronaviruses, which are already significantly more infectious than the average virus. SARS2 also has features that are the sort of features we might expect from a virus in the WIV, such as the furin cleavage site and competence at infecting cells through ACE2. WIV research definitely focused on the latter and possibly on the former, so this attribute of the outbreak also increases P(last bullet | WIV leak).

There is also one known precedent for (some pandemic outbreak | some lab leak) which is the 1977 H1N1 flu epidemic. It isn't known with certainty to be from a lab but there is a general consensus that it probably is.[4]

Sorry for not providing numerical estimates for any of these. That would feel super handwavy and they would be, like you admit, entirely unconvincing. Besides, any motivated person can adjust his priors until Bayes spits out the desired outcome. In any case, I feel that the WIV leak theory is simply a more parsimonious explanation of the particular attributes of this pandemic than the zoonotic origin theory. However I doubt there will ever be a "smoking gun" for either theory.

[0] https://en.wikipedia.org/wiki/List_of_laboratory_biosecurity...

[1] https://www.washingtonpost.com/opinions/2020/04/14/state-dep...

[2] https://www.youtube.com/watch?v=ovnUyTRMERI

[3] https://web.archive.org/web/20201206204844/https://www.scien...

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

[Symmetry]

For exposures not turning into pandemics, I think it's also important to note that most bat/human coronavirus crossovers occur in relatively small communities that limit the size of potential super spreader events. In something like the original form of Covid-19 where most people infected don't infect anyone else and the small number of people who infect hundreds are the main driver of the high R we can't necessarily generalize from what we see in rural Yunan and SE Asia to what we'd see in a big city. Though regardless of the origins of Covid-19 I do worry about these regions getting more connected to the global economy.

[jpeloquin]

Thanks for your reply. Not providing numerical estimates is fine with me, my opinion is that they're not productive anyway. It's like the Drake equation but worse, or trying to estimate the probability of divine creation vs. a natural origin for the universe. Too many unknowns, no ability to test counterfactuals. We are not even close to inventorying all relevant natural coronaviruses, so we don't have a good estimate for the number of "draws" to assume against the natural origin, and we don't do the routine monitoring that would allow a good estimate of probability for potentially pandemic pathogen → actual pandemic. So an armchair analyst ends up with a solid "maybe", with massive uncertainty intervals, for any explanation.

I completely agree that WIV's practices were dangerous and irresponsible, and that they are a plausible origin at all is a crushing condemnation. There's an old piece of advice: imagine that what you're doing ends up on the front page of the newspaper, and consider whether you would be proud or ashamed. If the latter, do something else. WIV clearly didn't follow that advice.

The facts in your post are all correct to my knowledge, I only quibble with the emphasis on infectiousness. The uniquely dangerous aspect of SARS-CoV-2 was its several-day period of transmissibility in the absence of symptoms, not its infectiousness, which for early strains was not that remarkable. Without asymptomatic transmission it would probably have been contained and quickly forgotten, like SARS-CoV-1. We need to adopt methods that will detect and contain viruses like SARS-CoV-2 so we're better prepared for when SARS-CoV-3 shows up.