The SARS-CoV-2 coronavirus, the covid virus, may have made the leap in species (spillover, in English) twice from animals to humans: this is what emerges from a new analysis (published on virological.com and still not peer-reviewed) of which an article in Nature speaks: the results, if confirmed, would discard definitively the laboratory hypothesis.
A – B. The first viral genomes of SARS-CoV-2, sequenced between late 2019 and early 2020, belong to two lineages (families), called A and B. The B lineage then became the dominant one globally, while the A has spread only in China. What is important to understand is whether the two lineages are connected – that is, if one is the evolution of the other – or whether they have separate origins. In the second case, the virus may have made more than one species leap, perhaps in several wet markets in Wuhan. “If it were shown that A and B are two separate lineages and that two spillovers have occurred, the laboratory hypothesis would be definitively ruled out,” emphasizes Robert Garry, a virologist at the University of New Orleans (USA).
Straight parallel. Out of 1,716 SARS-CoV-2 viral genomes analyzed, sequenced before February 28, 2020, the researchers identified 38 “intermediate” genomes, initially considered hypothetical evolutionary stages of the same virus. But looking at the genomes more closely, the researchers found that the mutations present in the different samples belonged to the A lineage. or to B, and not to an intermediate phase of evolution between the two lineages. “According to our analyzes, it is quite unlikely that one of the intermediate genomes is actually a transitional genome,” says Joel Wertheim, a molecular epidemiologist at the University of California at San Diego.
Nothing strange. The hypothesis is therefore that some animals belonging to one or more species have infected more people on various occasions, probably in different markets in Wuhan. If confirmed, the possibility that SARS-CoV-2 is the result of a double jump of species should not surprise anyway, since there are an infinite number of coronaviruses in nature: according to another study, every year about 400,000 people are infected with SARS-related coronaviruses without knowing it, through species leaps that (fortunately) do not turn into pandemics.