(1) After peer review, unscientific language was removed from the Worobey et al. manuscript. However, these strong claims in the preprint had already been widely reported in the media back in February 2022. Needless to say, those journalists likely will not be making corrections or publishing new stories to clarify that these excessive claims have now been eliminated by scientific peer review.

For example, both of the following strong assertions that were in the preprint have been revised; there are no longer claims of dispositive or incontrovertible evidence in the peer-reviewed paper:

“Together, 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.”

“Collectively, these results provide incontrovertible evidence that there was a clear conduit, via susceptible live mammals, for the zoonotic emergence of SARS-CoV-2 at the Huanan market towards the end of 2019.”

(2)The peer-reviewed paper has an entirely new section on “Study Limitations” which acknowledges that the scientists do not have access to the early Covid-19 case data or locations, lack direct evidence of a market animal infected with the pandemic virus, and lack complete details of how the market had been sampled for the virus.

Despite lacking access to data, Worobey et al. 2022 surprisingly claim in their preprint and the peer-reviewed paper that “positive environmental samples [were] linked both to live mammal sales and to human cases at the Huanan market.”

This stands in contrast to what the Chinese CDC claims in their own February 2022 preprint — that there was no link between positive environmental samples and the type of product sold by vendors. “The market might have acted as an amplifier due to the high number of visitors every day, causing many initially identified infection clusters in the early stage of the outbreak.”

Who is correct? Only the data can say. The Chinese CDC manuscript is still undergoing peer review. I hope that the journal and peer reviewers ask them to publish their complete dataset so that this question can be put to rest.

FYI the Worobey et al. Science article indicates that the supporting data for their analysis can be found in their preprint: “Data and materials availability: Data and code for this manuscript are available from (53).” Their preprint reveals that the data underlying their analysis came from two key sources: a media leak in June 2020 by the Epoch Times, and the China-WHO joint report in early 2021 which consisted of only information that had been provided by scientists in China.

(3)The paper still fails to acknowledge that early Covid-19 cases, regardless of whether the patient had a known link to the market, had been identified with ascertainment bias— reinforcing the initial perception that the Huanan seafood market was the early epicenter of the Wuhan outbreak.

Multiple reports from the early days of the pandemic and even the China-WHO joint study tell us that, due to the initial suspicion that the virus had spilled over from illegal wildlife sold at the market, the retrospective identification of December 2019 coronavirus cases included in their case definition a link to the Huanan market and early surveillance focused on hospitals near the market and the neighborhood of the market; this market bias was only removed from the criteria for identifying potential Covid-19 cases on January 18, 2020.

This cannot be stressed enough: Early cases with no links to the market had been identified almost solely through searching the hospitals near the market and the neighborhood of the market.This is why even the unlinked cases look like they cluster around the market.

Later retrospective searches that removed the requirement for a link or proximity to the market (or its neighborhood and nearby hospitals) for early Covid-19 cases in 2019 in Wuhan were reportedly unproductive (see the China-WHO joint study report).This means that, due to the ascertainment bias, early clusters and cases without links to the market, hospitals near the market, or the neighborhood of the market would have easily been missed.

At the very least, after peer review, the Worobey et al. 2022 paper now says: “However, the observation that the preponderance of early cases were linked to the Huanan market does not establish that the pandemic originated there.”At the very least, after peer review, the Worobey et al. 2022 paper now says: “However, the observation that the preponderance of early cases were linked to the Huanan market does not establish that the pandemic originated there.”

For most people, this is probably the first time you’re hearing about 2 strains of the pandemic virus that spilled over from animal to human in the Wuhan market.

Why haven’t you heard about this till now? Wasn’t it just one virus that emerged in Wuhan in late 2019?

This is because it was just the 1 virus (1 strain) that emerged in Wuhan in late 2019.

The so-called “2 strains” — one is called lineage A and the other is called lineage B — that you’re hearing about in the news today are the same virus but with only 2 mutations differentiating A and B. Consider that the virus genome is close to 30,000 letters long. It’s a bit of a stretch to say that A and B are two different strains. For context, the Alpha variant has 44 mutations compared to the original Wuhan virus; the Delta has 50 mutations compared to the Wuhan version; and the Omicron has 113 mutations compared to the Wuhan virus. Within a single infected individual, there are billions of virus particles, many of which carry different sets of mutations. If we start to separate “strains” of the virus by only 1–2 mutations, this would mean that every Covid-19 patient is carrying possibly thousands of different strains of the virus.

With only 2 mutations separating the “2 strains”, there is no need for more than one introduction of the virus into the human population to have occurred.Each time a person infects another person with the virus, there is already an estimated 5–10% chance the virus will pick up 2 mutations.For example, in theDiamond Princess cruise outbreak in early 2020, a single introduction of the virus into the cruise passenger population resulted in some of the virus picking up 5 mutations within ~3 weeks of the first infected person developing symptoms.

So it is much more likely that a single version of the virus was introduced into the human population in Wuhan, transmitted undetected, and quickly picked up 2 mutations — as opposed to the “2 strains 2 spillover” hypothesis proposed by Worobey and Pekar et al. 2022 Science, where there has to be 2 separate transmissions of the same virus from as yet still missing infected animals into people at the Wuhan market.Considering that no direct evidence exists for even a single zoonotic spillover at one market, a single introduction of the virus into the Wuhan human population is most consistent with the data available thus far.

So why are Worobey and Pekar et al. pitching a “2 strains 2 spillover” hypothesis if there was really only the 1 strain and no direct evidence of even a single spillover?

This is because the early cases at the Wuhan market were all only infected by the B version of the virus. So, the question is where did the A version of the virus come from? Furthermore, the A version is widely considered to have preceded the B version — indeed, it was named lineage A because it was presumed to come before lineage B–andseveral early patients who did not visit the market were found to be infected with A.For instance,one family from Shenzhenvisited Wuhan between 29 December and 4 January, and without any known exposure to the market picked up the A virus. (On a more technical note: It is very unlikely that A evolved from B because the virus is more likely to mutate away from the closest bat virus relative. There are so many possible mutations a virus can pick up to make it better at spreading in human and other animal species, why would it pick the 2 mutations that revert towards the bat version? See these two studies for more details.)


Under these circumstances, the scenario most strongly supported by the available genetic and epidemiological evidence is that the virus had started infecting people somewhere in the city, a patient carrying the (later) B version visited the market and sparked off the early superspreader event.Remember that the market, with a retail space of ~ 10 USA football fields, had an estimated ten thousand visitors a day, and was situated in one of the most densely populated and centrally located neighborhoods in Wuhan. It is also located close to several of the main hospitals in the city center of Wuhan. It is not a surprise that the poorly ventilated, crowded wet market would have been a prime location for an early Covid-19 superspreader event.

This obviously puts proponents of the Wuhan market origin in a tough spot. How can they explain that the available evidence doesn’t seem to point to the market being the original site of an animal-to-human or even the first human-to-human transmission of the virus?