The Logic of the COVID “Lab Leak" Hypothesis

The exact origins of SARS-CoV-2, the virus that causes COVID, still remains a mystery. Some people think this makes all possible origin stories equally probable. It does not. Facts about the virus itself tell us that it was not engineered, and experts agree that the "animal crossover" theory is most likely. On this theory, a virus mutated as it was passed from bats (probably through another animal and then) to humans. Despite this, the “lab leak” theory remains popular on social media. In light of recent reports that three Wuhan lab workers were hospitalized in November 2019, statistician Nate Silver tweeted that his estimation of the “lab leak” hypothesis jumped from 40 percent to 60 percent, meaning that he now thinks it is more likely than not.

But is that really reasonable? Let’s look at the logic of the argument and relevant evidence.

The scientific evidence against the lab leak hypothesis.

The lab leak hypothesis is a kind of middle ground between the intentionally engineered hypothesis and the animal crossover hypothesis. It doesn’t suggest that the virus was intentionally engineered for public release; it’s the idea that a natural virus being studied at the Wuhan Institute of Virology (WIV or “Wuhan Lab”) accidentally leaked to the outside world.

But the evidence against the lab leak hypothesis is pretty strong.

Simply put, we have a pretty good idea of what coronaviruses they were studying at the WIV, and none of them were similar enough to SARS-CoV-2 to be its origin. Of the viruses that were being studied at the WIV, RaTG13 is the closest genetic relative to SARS-CoV-2, but multiple experts agree: it would take 50 years for RaTG13 to naturally mutate into SARS-CoV-2. Since RaTG13 was only first discovered in 2013, obviously it cannot be the "parent" of SARS-CoV-2.

It’s possible that the WIV is lying about what viruses they were studying, and that the first identified outbreak of COVID happened close to the WIV makes some think that they must be. “Isn’t that just too much of a coincidence?” Not really, it turns out.

What are the odds?

First, it wouldn’t be surprising if a new, naturally occurring coronavirus emerged near a lab that studies them. Why? Because the viruses being nearby is why the lab studies them. The WIV is located in Wuhan, the largest developed Chinese city in the region where (scientists have warned us) coronavirus crossover events are likely to happen: Southeast Asia. (The horseshoe bats that harbor SARS-type viruses live near there, and SARS-CoV-1 reached humans by crossing over from those bats through other animals: Himalayan palm civets and raccoon dogs.) Thinking “the WIV must have caused the outbreak” is like thinking a scientific outpost studying the melting ice caps is what is causing them to melt.

To be fair, it wasn't just in the same region; the wet market linked to the first identified outbreak was just a few blocks down the road from the WIV. But that's just it; it was only the first identified outbreak. That the first identified outbreak was near the WIV doesn’t mean that the virus first infected humans there, or even that the first outbreak was there. That’s just where we first saw it. And it’s very likely that we first saw it near the WIV because the WIV was there to help us see it. Other (especially rural) places wouldn’t have had the luxury of having one of the world’s top viral institutes down the road, so an outbreak of a new disease could have easily gone unidentified.

Thinking that the virus started near the WIV because we first saw it near the WIV is like thinking the person who just stepped into the spotlight on stage came into existence as he stepped into the spotlight. The WIV is like a viral spotlight; SARS-CoV-2 could have been circulating around China for weeks or even months before the December 2019 Wuhan outbreak, but we were only able to see or detect it (as a new virus) once it hit a location with a hospital that had easy access to the WIV. (This is related to the streetlight fallacy.)

Third, there is reason to think that this is exactly what happened. We now know that

1. The December 2019 meat market outbreak was just the tip of the iceberg, with more than 1000 cases throughout the city alongside the (initially reported) 174 severe cases linked to market.
2. There were already multiple (13) variants in Wuhan by December 2019 (meaning that the virus was circulating at least as early as October 2019).
3. Someone was infected with COVID elsewhere in (rural) Hubei province as far back as November 17, 2019.

Not only do we now know that the Wuhan wet market was not the crossover event, we know SARS-CoV-2 was circulating outside Wuhan before it was finally identified there by the WIV. This makes the Wuhan lab leak theory much less likely.

Hospitalized lab workers

What about those three WIV lab workers who went to the hospital in November 2019? At least three things are relevant here.

1. “[C]urrent and former officials familiar with the intelligence expressed a range of views about the strength of the report's supporting evidence, with one unnamed person saying it needed 'further investigation and additional corroboration'." It’s not even clear if the hospitalizations happened. (It contradicts other reporting that said all the WIV workers tested negative for COVID antibodies.)
2. The report doesn’t even say what they went to the hospital for. If they weren’t all there for COVID-like symptoms, this evidence is irrelevant.
3. We’d also need to know: three out of how many workers? How unlikely would it be, at any given time, during that time of the year, for three WIV workers to be in the hospital? Is this an aberration, or a common occurrence?

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Honestly, without answers to the above questions, this “report” shouldn’t affect, at all, how likely anyone thinks the lab leak hypothesis is. I’m not sure how Nate Silver did his calculation—he doesn’t show his work, or how he filled in his other values. (How likely does Nate think the hospitalization evidence is on the lab leak hypothesis, and why?) But given what we have considered, his initial prior probability for the lab leak hypothesis should have been much lower than 40 percent, and this unconfirmed vague reporting shouldn’t have affected it at all.

The case is not closed

To be clear, I’m not saying that the lab leak hypothesis is crazy, has been completely ruled out, or that there shouldn’t be a more transparent investigation into the origins of SARS-CoV-2. After all, lab leaks have happened before, worries about the WIV were expressed in 2018, China doesn't have the best track record when it comes to being open and forthcoming about this matter, and the knockdown evidence against the lab leak hypothesis still eludes us: finding the animal reservoir. (Although this would be a middle ground between the lab leak and crossover theories—since direct contamination is possible (but unlikely), maybe a lab worker just got infected while collecting samples from bats.)

What's more, I found one study that suggests the reported 17 percent divergence between SARS-CoV-2 and RaTG13 (that virus beings studied at the WIV) might have been underestimated—although the study has admitted flaws and doesn't say what a better estimate would be. (By comparison, RmYN02, which is found in horseshoe bats in Yunnan, China, is only 6 percent divergent.)

But given what we know now, the lab leak hypothesis remains unlikely. I worry greatly, not only about how seriously people are taking it but about how people are even rooting for it to be true. Why? As Forbes science writer Ethan Seigel put it:

[T]he reason so many people have died is because of a global political failure to respond appropriately. Attempting to shift the blame onto the very scientists who have been instrumental in understanding and combating the virus is a tactic straight out of Operation Himmler, and must be opposed by the entire scientific community in full force.

I couldn’t have put it better myself.

Copyright 2021, David Kyle Johnson