From the beginning of the Covid-19 pandemic, it was evident that SARS-CoV2, the agent of Covid-19, was more contagious than other well-known coronaviruses that cause SARS or MERS. Based on a fascinating “shell disorder” model, the reason may lie in the “odd” combination of “hardiness” of its membrane protein (M) (outer shell) making it more likely to survive in body fluids and environment, and resilience of its nuclear protein (N) (inner shell) making it more likely to rapidly replicate even before the immune system detects it.¹

Outer shell hardiness of the M protein of SARS-CoV2 contributes to its persistence in the environment and resistance to digestive enzymes in saliva, mucus, stool, and other bodily fluids. Inner shell resilience of the N protein can lead to greater virulence through more rapid replication of viral proteins and particles. The latter is also an efficient way of evading the host immune system ie, by the time the immune system finds out there is a problem, the virus has already reproduced in high numbers in the absence of symptoms!

Long before Covid-19 pandemic, a group of scientists proposed categorization of coronaviruses into 3 major “shell disorder” categories (based on the features of the M and N proteins), correlating with their primary modes of transmission. Category A: higher levels of respiratory transmission, lower levels of fecal-oral transmission (eg. HCoV-229E, common cold coronavirus); category B: intermediate levels of respiratory and fecal-oral transmission (eg, SARS-CoV); and category C: lower levels of respiratory transmission with higher levels of fecal-oral transmission (eg, MERS).^1,2  

It turns out that Covid-19 falls into category B which means that it has the potential for transmission not only through respiratory route but also through fecal-oral route and the environment. What’s “odd” about SARS-CoV2 though is that it seems to have the hardiest outer shell compared to SARS-CoV and other coronaviruses in its category.

So not only is Covid-19 more likely to be transmitted due to high viral loads in the respiratory tract even before symptoms develop, it may have an advantage over other respiratory coronaviruses by persisting in the environment when contaminated by respiratory secretions, feces or other body fluids.

Truly a “novel” virus!

Bonus Pearl: Did you know that despite being more contagious, Covid-19 is fortunately less fatal than SARS or MERS?

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References

1. Goh GKM, Dunker AK, Foster JA, Uversy VN. Shell disorder analysis predicts greater resilience of the SARS-CoV-2 (COVID-19) outside the body and in body fluids. Microbial pathogenesis 2020;144:104177. https://pubmed.ncbi.nlm.nih.gov/32244041/
2. Goh GKM, Dunker AK, Uversky VN. Understanding viral transmission behavior via protein intrinsic disorder prediction: Coronaviruses. J Pathol 2012;2012:738590. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3477565/

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