Why does SARS-CoV-2 spread so fast?

The world has been hard-hit by COIVD-19 pandemic caused by SARS-CoV-2 since the end of last year. To date, due to the swift spread of the virus, 16,882,529 confirmed cases have been reported within 8 months, the number is much bigger than that of SARS (8090 cases) in 2003. Why does the new coronavirus spread so fast? 

To begin with, structural biologists reveal that the host cell entry of SARS-CoV-2 depends on the same receptor ACE2 as SARS-CoV. However, the spike protein of SARS-CoV-2 binds with ACE2 at least 10 times more tightly than the corresponding spike protein of SARS-CoV to the protein. As a result, the minimum infectious dose of SARS-CoV-2 is much lower than SARS-CoV. In another word, less amount of SARS-CoV-2 are needed to make a person sick than that of SARS-CoV. 

 SARS-CoV-2 S binds human ACE2 with high affinity (Obtained from Science, 2020)

Moreover, researchers have found the high aerosol and surface stability of SARS-CoV-2. A research conducted by Vincent Munster’s team have shown that the virus can remain viable and infectious in the air for at least 3 hours and on surfaces (plastic, stainless steel, copper, and cardboard) for as long as 3 days.

Viability of SARS-CoV-1 and SARS-CoV-2 in Aerosols and on Various Surfaces (Obtained from the New England Journal of Medicine, 2020)

With high affinity of spike protein and long survival time, SARS-CoV-2 can be easily exposed to people and create infections. Scientists speculate that those two factors may be main biological reasons for the fast pandemic spread.

References

Wrapp, D., Wang, N., Corbett, K. S., Goldsmith, J. A., Hsieh, C. L., Abiona, O., ... & McLellan, J. S. (2020). Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation. Science, 367(6483), 1260-1263.

Van Doremalen, Neeltje, et al. "Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1." New England Journal of Medicine 382.16 (2020): 1564-1567.