Where is SARS-CoV-2 from?

Since the late December 2019, the world has been badly struck by the sudden outbreak of SARS-CoV-2. Despite of the uncertainty towards the source of SARS-CoV-2, two scenarios have been developed by the scientists involved in epidemiological studies. 

Scenario I: a virus directly came from wildlife

Wildlife is critically important for ecosystems around the world. But they also harbor 70% of emerging infectious diseases including some of the toughest known zoonoses such as West Nile fever, Lyme disease, Rabies and Ebola. Given that SARS-CoV-2 is a part of coronaviruses family, it was likely to be naturally hosted and evolutionarily shaped by wild bats. Zhengli Shi and her colleagues reported that SARS-CoV-2 genomes share 96% sequence similarity with a bat coronavirus, indicating it is probably spilled over from bat. As of June 29, there is still however no evidence of direct bat-human transmission, suggesting a likely participation of an intermediate host. A study published on March 26 showed that the receptor-binding domain (RBD) of SARS-CoV-2 is almost identical to that of the Guangdong pangolin coronaviruses (97.4% amino acid similarity). Thus, pangolin may play a role in the process through which the virus jumped to humans. Nevertheless, rather than a pangolin directly to human, it could also be transferred from a pangolin to a second intermediate host and that animal was actually where there was human contact and infection.

A pangolin on a man's hands (Marketplace.org)

Scenario II: a virus evolved among humans

Some China's earliest COVID-19 cases were connected to the Huanan Seafood Wholesale Market in Wuhan, where illegal pangolin trade could happen. It is likely the seafood market played a role in transferring the virus since some people who visited the market developed viral pneumonia caused by the new coronavirus. However, there is evidence emerging that among the first 41 patients hospitalized in China, 13 had no connection to the market, suggesting a possibility of initial human-human transmission. The underlying event that gave the virus pathogenic genomic features could occur at any time during the transmission period. In other words, SARS-CoV-2 possibly evolved through adaptation during unrecognized human-to-human transmission, especially given the fact that the first confirmed case was reported about one month later after the earliest estimated occurrence of the common ancestor of the virus. 

Nobody can travel back in time to see how exactly the virus occurred. As a result, those two scenarios developed by researchers were both built on some sort of assumptions. Nevertheless, that doesn’t mean scientific research is meaningless. After all, science is not about knowing the absolutely truth, it is about pursuing infinitely approach to the truth. The origin of the virus remains a myth, but we can say with very high certainty that it is a result of natural selection rather than artificial genetic modification since it owns a unique molecular backbone that substantially differ from those of any known infectious viruses. There is no doubt that SARS-CoV-2 research findings will help in the prevention of future zoonotic events.

References

Andersen, K. G., Rambaut, A., Lipkin, W. I., Holmes, E. C., & Garry, R. F. (2020). The proximal origin of SARS-CoV-2. Nature medicine, 26(4), 450-452.
Lam, T. T. Y., Jia, N., Zhang, Y. W., Shum, M. H. H., Jiang, J. F., Zhu, H. C., ... & Li, W. J. (2020). Identifying SARS-CoV-2-related coronaviruses in Malayan pangolins. Nature, 1-4. 
Tang, X., Wu, C., Li, X., Song, Y., Yao, X., Wu, X., ... & Cui, J. (2020). On the origin and continuing evolution of SARS-CoV-2. National Science Review.
Zhou, P., Yang, X. L., Wang, X. G., Hu, B., Zhang, L., Zhang, W., ... & Chen, H. D. (2020). A pneumonia outbreak associated with a new coronavirus of probable bat origin. nature, 579(7798), 270-273.



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