Two new studies published in the journal Science again point to the wuhan marketin China, as epicenter of the appearance of SARS-CoV-2, from activities associated with wild animal trade.
The investigations, led by Michael Worobey and Jonathan Pekar, use complementary approaches involving spatial and environmental analyses, as well as molecular studies to provide evidence that the Huanan market in Wuhan was the early epicenter of the covid-19 pandemic.
They conclude that it is very likely that SARS-CoV-2 was present in living mammals that were sold in this market at the end of 2019 and also suggest that the genomic diversity of SARS-CoV-2 before February 2020 probably comprised only two lineages distinct viralcalled A and B.
These lineages were the result of at least two separate events of interspecies transmission to humans.
The study
To test the hypothesis that the market was the epicenter of the pandemicWorobey and his team at the University of Arizona obtained data from various sources.
First, they used cartographic tools to estimate location in longitude and latitude of more than 150 of the first reported cases of the virus as of December 2019, including those that had not been reported as directly linked to the market.
The highest density of these cases was centered around Huanan marketthey say
Mapping cases from January and February 2020 using data from Weibo, a social media app that created a channel for people with Covid to seek medical help, the researchers cases identified in other parts of central Wuhan radiating from the market as the pandemic progresses.
In subsequent analyzesscientists report that in the Huanan market multiple potential hosts sold -of the virus- wildlife intermediates until at least November 2019.
Using and expanding a dataset on surface samples from the aforementioned market, they identified five stalls that likely sold live or freshly butchered mammals; according to the analyses, proximity to vendors of these live mammals was predictive of human virus cases.
For their part, to better understand the circumstances that led to the origin of the pandemic, Pekar and his group at the University of California San Diego analyzed the genomic diversity of SARS-CoV-2 at the beginning of the pandemic.
Although the diversity of the coronavirus increased to As the pandemic spread from China to other countriestwo lineages of SARS-CoV-2 -called A and B- marked the start of the pandemic in Wuhan.
Another possibility
It has been hypothesized that the two lineages arose separately. To test it, the team analyzed genomic and epidemiological data from the first moments of the pandemic with models and simulations.
Their conclusion is that the first zoonotic transmission likely involved lineage B viruses around November 18, 2019 (never before the end of October), while the separate introduction of lineage A likely occurred days-weeks after this event.
As with other coronaviruses, the appearance of SARS-CoV-2 was probably the result of multiple zoonotic events.
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The results of the Worobey study are also consistent with this idea of a separate origin and the subsequent appearance of lineage A in the Huanan market in late November 2019.
These findings indicate that SARS-CoV-2 is unlikely to have circulated widely in humans before November 2019 and define the narrow window between the time SARS-CoV-2 first jumped to humans and when the first covid cases were reported.