“Zoonotic,” in case you didn’t know, refers to an infectious disease transmitted between animals. And, in a post a few days ago, I highlighted a paper in Science suggesting that the coronavirus did originate as a zoonotic disease: it came from horseshoe bats and was transferred by bats to another mammal (one likely candidate is the palm civet or “civet cat”, a viverrid, not a felid), and then from this mammalian carrier to humans in Wuhan “wet markets.” The authors emphasized that there was no evidence that the virus came out of the local lab.
A new paper in press in Cell comes to the same conclusion, though they summarize all the evidence, not just the phylogenetic evidence (family tree of viruses). The new paper, however, is not as certain about the species of mammal that transferred the virus from bat to human. But they are pretty sure that the virus was not cultured in the Wuhan Institute of Virology (WIV), and then either escaped or was somehow released to cause disease. This paper, which Matthew called to my attention, has an international team of distinguished disease experts as authors, and they summarize all the evidence that COVID-19 is a purely zoonotic disease and escaped from a Wuhan wet market, not from the WIV. The paper is really only 11 pages long, and you can download the pdf by clicking on the screenshot below.
I’ll just summarize the lines of evidence (there’s more in the paper, too, but the first 11 pages of double-spaced text is all you need to read.
1.) All previous coronavirus infections of humans (viruses other than SARS-CoV2, or what I’ll call COVID-19) have a zoonotic origin, several of which had their origin in “wet markets” selling animals like civet cats and raccoon dogs. Workers in these markets have high concentrations of antibodies against various coronaviruses.
2.) The sequence of COVID-19 is similar to that of other coronaviruses in humans known to have zoonotic origins.
3.) Epidemiologically, the spread of the virus strongly implicates the wet market in Wuhan as the source, not the WIV. As the authors note:
Based on epidemiological data, the Huanan market in Wuhan was an early and major epicenter of SARS-CoV-2 infection. Two of the three earliest documented COVID-19 cases were directly linked to this market selling wild animals, as were 28% of all cases reported in December 2019 (WHO, 2021). Overall, 55% of cases during December 2019 had an exposure to either the Huanan or other markets in Wuhan, with these cases more prevalent in the first half of that month (WHO, 2021). Examination of the locations of early cases shows that most cluster around the Huanan market, located north of the Yangtze river (Figure 1B-E), although case reporting may be subject to sampling biases reflecting the density and age structure of the population in central Wuhan, and exact location of some early cases is uncertain. These districts were also the first to exhibit excess pneumonia deaths in January 2020 (Figure 1F-H), a metric that is less susceptible to the potential biases associated with case reporting. There is no epidemiological link to any other locality in Wuhan, including the Wuhan Institute of Virology (WIV) located south of the Yangtze and the subject of considerable speculation. Although some early cases do not have a direct epidemiological link to a market (WHO, 2021), this is expected given high rates of asymptomatic transmission and undocumented secondary transmission events, and was similarly observed in early SARS-CoV cases in Foshan (Xu et al., 2004).
If you’re a conspiracy theorist that the virus was released from the lab by mistake, you’d have to say that it somehow got itself over to the wet market before it started infecting people. The wet market, not the WIV, was the epicenter of the infection.
4.) The COVID-19 virus was actually detected in “environmental samples” taken in the Hunan wet market, especially in the part of the market that sold animals and animal parts.
5.) As I showed in my post two days ago, the viruses closest in sequence to the human COVID-19 virus are three bat viruses from Yunnan. (It’s still not clear how they or their relatives found their way to the Yunnan wet market). But the telling part is, as the authors say, “None of these three closer viruses were collected by the WIV and all were sequenced after the pandemic had begun.”
6.) The absence of the known intermediate animal host for COVID-19 does not suggest that the virus was clearly engineered by humans in the lab, for the animal source of many human pathogens of zoonotic origin, including Hepatitis-C, polio, and Ebola, have not been identified.
7.) Although there have been isolated incidents in labs in which people got infected with viruses, there’s been only one documented example of a pandemic coming from human origin: “the 1977 A/H1N1 flu epidemic, that most likely originated from a large-scale vaccine challenge trial.” There are no epidemics known caused by the escape of a novel virus. (You might respond that, “Well, this could be the first one,” but the other evidence I adduce tells against this.)
8.) There is no evidence that the WIV or any other lab was working on the SARS-CoV-2 virus or any related virus before the pandemic.
9.) Despite extensive attempts to find the virus in workers at the WIV, there are no reports of COVID-19 infections in that institute.
10.) Previous experimental work on coronaviruses at the WIV have involved inserting a “genetic backbone” and other genetic markers that we do not see in the human COVID-19 virus that’s causing the pandemic.
11.) To culture the virus in the lab, workers would have to infect wild-type mice, but were unable to do so with SARS-CoV-2. The virus has since been engineered to be culture-able in mice, but that occurred after the pandemic had already begun.
12.) Adaptive mutations that enhanced the infectivity of the virus arose after the pandemic started, ergo were not engineered in the lab.
13.) Sequences that “lab-contaminant” advocates say could only have been engineered into the virus by humans have in fact been found naturally in other coronaviruses. That they’re missing in close relatives of the coronavirus could reflect only our pretty profound ignorance of what strains SARS-CoV-2 evolved from from. And there is no evidence that that kind of genetic engineering was ever going on at WIV.
The “conclusions” on pp. 10-11 are pretty clear:
“the most parsimonious explanation for the origin of SARS-CoV-2 is a zoonotic event” involving transfer from an intermediate host in a Wuhan wet market.
“There is currently no evidence that SARS-CoV-2 has a laboratory origin.”
And the last paragraph:
We contend that although the animal reservoir for SARS-CoV-2 has not been identified and the key species may not have been tested, in contrast to other scenarios there is substantial body of scientific evidence supporting a zoonotic origin. While the possibility of a laboratory accident cannot be entirely dismissed, and may be near impossible to falsify, this conduit for emergence is highly unlikely relative to the numerous and repeated human-animal contacts that occur routinely in the wildlife trade. Failure to comprehensively investigate the zoonotic origin through collaborative and carefully coordinated studies would leave the world vulnerable to future pandemics arising from the same human activities that have repeatedly put us on a collision course with novel viruses.
This paper is of course tentative, like all such conclusions, but the data add up to a “normal” zoonotic event and not escape from the lab. It’s clear the virus was not engineered to kill humans as a bioweapon, as there’s no evidence that the WIV worked on it. And even if it did, why would it happen to escape to a wet market—places where these viruses are known to exist naturally. Nor is there evidence that the WIV was simply studying the virus and it escaped as an accident that caused the pandemic.
In other words, conspiracy theories about the virus seem to be untenable, but, humans being human and prone to conspiracies, they’ll persist.
UPDATE: in the thread after this tweet, third author Rasmussen goes through the evidence that people think supports a lab origin, and then dispels it:
Today our review of the evidence for the origins of SARS-CoV-2 graduated from a pre-print to peer-reviewed pre-proof in @CellCellPress.
Can we rule out a "lab leak"? No, but if we objectively follow the evidence, it leads us away from that hypothesis.https://t.co/IF2jAZ7BQE
— Dr. Angela Rasmussen (@angie_rasmussen) August 19, 2021