Where Did COVID-19 Come From?

Yesterday we reached a million cases, twice as last Friday. It’s hard to imagine that this now global disease was initially restricted to a single food market. But did the virus really originate there?

Photo: Carlos Jaimes.

The origin of the SARS-Cov-2 has been a constant source of debate and controversy. Everyone seems to have their own theory, which changes depending on where they stand in the ideological spectrum: Nicolás Maduro has defended the idea the virus was created in an undisclosed U.S. military facility to wage biological war against China; for other conspiracy theorists, the fact Chinese scientists have collected bats near Wuhan in the past is irrefutable proof of their role behind its creation.

Truth is, there’s strong evidence supporting that the virus evolved naturally from animals. Coronaviruses use a particular protein, called the spike protein, to bind to another protein (known as “receptor”) in human cells and infect them. The tridimensional configuration of this spike protein is different from those of previously known coronaviruses, and also from previously predicted models. For it to be a lab product, COVID-19’s creators would have had to use one of the already known configurations. Data also strongly suggests that this new configuration is the result of natural selection in animal reservoirs with a receptor similar to that of humans.

Truth is, there’s strong evidence supporting that the virus evolved naturally from animals.

The genetic code of SARS-Cov-2 is 96% identical to that of another coronavirus known to infect bats, which are probably the virus’ natural animal reservoir. The segment of the spike protein of the bat coronavirus that binds to the receptor, however, is different to that of SARS-Cov-2, suggesting that the virus didn’t jump directly from bats to humans, but rather infected an intermediary animal first. Pangolins are likely candidates, since several coronaviruses known to infect them have spike proteins practically identical to those of the new human coronavirus. The rest of the virus genes are not similar enough to those of SARS-Cov-2 to confirm they’re the intermediate host.

Pangolins are commonly poached to be used in Chinese traditional medicine.

Based on the study of different SARS-Cov-2 strains, which are similar but not entirely identical to each other, it’s possible to track their last common ancestor. This kind of phylogenetic analysis revealed the first human strain probably originated in November, 2019. 

Until early January, all confirmed cases in China had visited the market and it was thought that an animal brought there originated the outbreak. However, it was later known that a patient who started presenting symptoms on December 1st, but was only diagnosed later, had no contact with the market. Considering the two week incubation period of the disease, this patient must have been infected in mid November, just when the virus jumped to humans.

This data suggests that upon its passage to humans,  the virus spread silently in a small cluster of patients and was then brought to the market, from where it quickly spread.

Even though details are not entirely clear, two things are certain: The virus was not created in a lab and it probably originated from a bat strain, from which it jumped to another animal, where it further mutated until being able to infect humans. In any case, this pandemic might not be an example of far-fetched biological warfare, but it does show the unforeseen consequences of poaching and illegal hunting of wildlife.