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Almost a year after the SARS-CoV-2 coronavirus was identified, scientists still have not determined how it could have been transmitted to humans. Virologist Étienne Decroly summarizes various hypotheses, including the hypothesis about an accidental escape from the laboratory.
Now that we are witnessing a race to develop vaccines or treatments, why is it so important to know the genealogy of the virus that causes the Covid-19 pandemic?
Étienne Decroly: SARS-CoV-2, which was quickly identified as the virus causing Covid-19, is, after SARS-CoV in 2002 and MERS-CoV in 2012, the third human respiratory syndrome coronavirus to emerge in the last twenty years. We are now well aware of this family of viruses that circulate mainly in bats and which zoonotic transfer occasionally causes human epidemics. Therefore, it is important to understand how the virus crossed the species barrier and became highly capable of human-to-human transmission. Studying the evolutionary mechanisms and molecular processes involved in the emergence of this pandemic virus is essential to better protect against the potential emergence of these viruses and to develop therapeutic and vaccine strategies.
From the first weeks of the pandemic, when little was known about the virus, its possible animal origin was quickly pointed out. Why was this piece selected from the beginning and has it been confirmed since then?
HIS. RE. The zoonotic origin of the coronaviruses, which infect nearly 500 species of bats, has already been well documented from previous appearances. In nature, bat populations share the same caves, and different virus strains can then simultaneously infect the same animal, which favors genetic recombination between viruses and their evolution. Some strains are sometimes able to cross the species barrier.
A group of bats in a cave in Burma. Almost 500 species of bats are infected with the coronavirus.
By comparing the genomic sequences of virus samples from different SARS-CoV-2 infected patients, an identity ratio of 99.98% was observed, showing that this virus strain appeared very recently in humans. It was also quickly discovered that this genome was 96% identical to that of the bat virus (RaTG13) collected in 2013 from animal faeces and whose sequences are only known from March 2020. We also noticed that the sequence of this genome was completely identical to fragment 370 nucleotides sequenced in 2016 from samples taken in 2013 at a mine in Yunnan Province, where three miners suffered severe pneumonia.
SARS-CoV-2 is not derived from known human strains and has only recently gained the ability to leave its natural animal reservoir.
Moreover, when analyzing the sequences of other known human coronaviruses, only 79% of genetic identity was found between SARS-CoV-1 and SARS-CoV-2 and only 50% for MERS-CoV. In short, SARS-CoV-2 is genetically closer to the strains of the virus that until now have only been transmitted between bats. It does not come from known human strains, and has only recently gained the ability to emerge from its natural reservoir, which is probably a bat.
If it is established that Covid-19 comes to us from the bat, why is its origin a matter of controversy?