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Covid: the stages of the virus and the intermediate host

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OF SURE, there is only the origin of the pandemic: established in Wuhan, the metropolis of the Chinese state of Hubei where the first cases of Covid-19 were recorded 15 months ago. Of the first stages of the health emergency that we have been carrying with us for a year, however, much remains to be defined. From the first “host” of the virus to the possible presence of intermediate animals through which Sars-CoV-2 has made its way to humans. Crucial aspects to define the origin of an infectious disease then extended on a large scale, without which “all the hypotheses remain on the table”, is what was reiterated by the director general of the World Health Organization, Tedros Ghebreyesus, at the end of the survey conducted in China by a pool of 34 experts from 11 different countries who carried out the mission in Wuhan for a month. With results at the moment interlocutors.


Covid: the origin of the pandemic

by Davide Michielin

The report prepared by international experts appointed by the World Health Organization and their Chinese counterparts after the visit in the first outbreak of the pandemic estimated it as “very likely” that Covid-19 has passed from bats to humans via an intermediate animal. And “extremely unlikely” that the virus escaped from a laboratory, considering that Sars-CoV-2 had never been revealed before in scientific articles nor was it sequenced by any laboratory in the world. The scientists’ work was “important”, but they limited themselves to classifying a series of hypotheses according to their degree of reliability. Among these, the one that would see an intermediate host between the bat and the man, as a link through which Sars-CoV-2 has come to manifest its most devastating effects.

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The investigations involved dozens of wild animals and livestock samples, but did not allow for a definitive conclusion. The origin of the pandemic from the Wuhan market is almost certain, given the high number of visitors infected in December 2019. “The epidemiological investigation leaves little room for doubt”, is the thought of Virginie Broker, evolutionist of the Jacque Monod Institute in Paris, reported on the columns of Nature.

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Reconstructing the stages traveled by the virus before reaching man is – in all likelihood – the most difficult mission. In fact, the sampling of wild animals marketed at the Wuhan wholesale market is very large, where the first cases of Covid-19 have been recorded. Follow the animals, is the mantra that circulates among virologists around the world. That is: following the animals to trace the origins of the pandemic. A complex and particularly expensive operation that should start with bats, probably the first to be infected with the coronavirus. And which, however, deserves to be extended to frozen meats, given that at the moment the hypothesis of a transmission through them has not been excluded.


Covid: trial of bats

by Simone Valesini

Second Linfa Wang, a virologist at Duke University in Singapore, “the investigation should also be extended outside of China, for example in Thailand and Cambodia”, where close relatives of Sars-CoV-2 have recently been isolated. In parallel, work could be done on blood samples stored for two at the Wuhan Blood Center, a blood bank from which useful information could be drawn about the presence of antibodies already before the end of 2019. A circumstance which, if confirmed, would provide a clue moreover relative to the moment in which the virus began to circulate in humans as well. This information is considered useful for reconstructing the origin of the pandemic, but also for preventing any new spillover from other viruses. “The paradigm must be changed: rapid tests and protective measures must give way to prevention at the source,” he explains David Heymann, epidemiologist at the School of Hygiene and Tropical Medicine at the University of London.

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The limits of the investigation

On this basis, experts are not sure that the first steps taken by Sars-CoV-2 will be learned before making its way around the world. “Tracing the origin of a virus can take years of work,” says David Robertson, a virologist at the University of Glasgow. According to the expert, the first step to take is an extensive and widespread sampling on animals, in order to possibly “trace the place and species from which the virus spread.” In the background remains the World Health Organization, in the crosshairs since the beginning of the pandemic for the delays with which it moved and for the reticence that accompanied all its steps. Including the latter. According to the United States, for example, the latest survey did not provide raw data on which to make researchers from all over the world work. Without neglecting the four-week limit and the narrow scope of the work, accepted obtorto neck by China. “The epidemiological study was managed as a diplomatic mission rather than a scientific investigation wide-ranging independent ”, is the comment of Nikolai Petrovsky, immunologist at Flinders University in Adelaide (Australia).


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