Why is Covid-19 more contagious than SARS or MERS?

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/

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Massachusetts General Hospital, Harvard Catalyst, Harvard University, its affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

Why is Covid-19 more contagious than SARS or MERS?

What’s the evidence that respiratory viruses, including Covid-19, can be transmitted by touching contaminated surfaces?

Although no published data specific to Covid-19 is yet available, transmission by contact with contaminated surfaces has been implicated in infections due to several respiratory viruses, such as other human coronaviruses and influenza viruses. 1,2

A 2020 review article involving 22 published studies found that human coronaviruses such as SARS, MERS or common cold coronaviruses (eg, HCoV-229E) can persist on inanimate surfaces (eg, metal, glass or plastic) for hours up to 9 days depending on the level of initial viral contamination.1  

A recent NEJM study reported Covid-19 persisting  for 72 h on plastic and 48 h on stainless steel (3). Shorter survival was observed on cardboard (24 h or less) and copper surface (4 h or less). Although data on transmissibility of coronaviruses from contaminated surfaces to hands is not currently available, at least in the case of influenza A, a contact time of 5 seconds may transfer 31.6% of the viral load to the hands.4

But hand contamination doesn’t necessarily stop there.  We constantly touch our faces, including nose, eyes, and mouth, all serving as potential entry points for the virus.   One study found that, on average, subjects touched their faces 23 times per hour, with nearly one-half of that time involving either the nose, eyes or mouth. 5 Another study reported touching one’s face on average 19 times in a 2-hour period (range 0-105 times!).

For these reasons, environmental decontamination and hand hygiene have been stressed as part of the ongoing strategies to limit Covid-19 spread.

The good news is that coronaviruses are efficiently inactivated by many of the commonly available disinfectants and antiseptics, including 62%-71% ethanol, 70% isopropyl alcohol, 1:50 dilution of household bleach, and 0.5% hydrogen peroxide. 1

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References

  1. Kampf G, Todt D, Pfaender S, et al. Persistence of coronavirus on inanimate surfaces and their inactivation with biocidal agents. J Hosp Infect 2020;104:246-51. https://www.ncbi.nlm.nih.gov/pubmed/32035997
  2. Otter JA, Donskey C, Yezli S, et al. Transmission of SARS and MERS coronaviruses and influenza virus in healthcare settings: the possible role of dry surface contamination. J Hosp Infect 2016;92:235-250. https://www.ncbi.nlm.nih.gov/pubmed/26597631/
  3.  van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med 2020, March 17. https://www.nejm.org/doi/10.1056/NEJMc2004973
  4. Bean B, Moore BM, Sterner B, et al. Survival of influenza viruses on environmental surfaces. J Infect Dis 1982;146:47-51. https://www.ncbi.nlm.nih.gov/pubmed/6282993
  5. Kwok YL, Garlton J, McLaws ML. Face touching: a frequent habit that has implications for hand hygiene. Am J Infect Control 2015;43:112-4. https://www.ncbi.nlm.nih.gov/pubmed/25637115
  6. Elder NC, Sawyer W, Pallerla H, et al. Hand hygiene and face touching in family medicine offices: a Cincinnati Area Research and Improvement group (CARInG) Network Study. J Am Board Fam Med 2014;27:339-346. https://www.jabfm.org/content/27/3/339.long
What’s the evidence that respiratory viruses, including Covid-19, can be transmitted by touching contaminated surfaces?