Understanding the Mode and Dynamics of SARS-CoV-2 Transmission

Idris Nasir Abdullahi

Severe Acute Respiratory Syndrome Coronavirus -2 (SARS-CoV-2) is the third highly pathogenic coronaviruses that has affected human race in the 21st century. As the incidence and fatality rates of SARS-CoV-2, the causal agent of coronavirus disease 2019 (COVID-19), continues to rise across 210 countries and territories, several preventive and control measures have adopted to halt the spread of SARS-CoV-2 and minimize COVID-19-associated death. As at 7:00AM GMT+1, 14th April, 2020, there were over 1925532 confirmed cases of SARS-CoV-2 infections with case fatality rate of 6.2%. [1] In Nigeria, there were 343 cases and CFR of 2.9%. [1]

For now, there is no available preventive or therapeutic vaccine and antivirals, hence the only means of controlling the spread of SARS-CoV-2 are through consistent adherence to physical distancing, personal hygiene and strict quarantine (or monitored self-isolation) of infected persons (or case contacts). For these measures to be successfully implemented and observed by everybody, there is a need for people to have good understanding of the mode and dynamics of SARS-CoV-2 transmission and clinical symptoms.

In order to accurately define suspicious cases of COVID-19, there is need to understand the hallmark of SARS-CoV-2-associated symptoms which include dry cough, fever, myalgia/arthralgia, headache, and gastrointestinal disturbance (in some instances). [2] Indeed, our understanding of the transmission of SARS-CoV-2 continues to improve with the evolution of the outbreak. As the COVID-19 pandemic continues to evolve, we are learning more about this novel virus. Based on available evidence on transmission of SARS-CoV-2, it can be broadly categorized into three epidemiological entities, viz; from being asymptomatic to pre-symptomatic and symptomatic. [3] It is worthy to note that all these categories of people are infected with SARS-CoV-2.

Briefly, a symptomatic COVID-19 case is one who has developed symptoms compatible with the definition of COVID-19 virus infection (as mentioned above). Whereas, symptomatic transmission refers to the transmission of SARS-CoV-2 from a person to previously uninfected people. This occurs through the nano-particles of respiratory droplets which can spread, contaminate surfaces and hands where they remain stable for hours. [4] SARS-CoV-2 contaminated hands act as mechanical vector. Thus, a potential site to terminate and prevent it from invading the body. However, if the virus is not eliminated at this stage, it can move towards its predilection site (i.e. cells of the lungs) where it attack cells of the respiratory tract. A SARS-CoV-2 infected individual has the ability to infect an average of three uninfected persons. [4]

Based on data from epidemiology and virologic studies on COVID-19, SARS-CoV-2 is primarily transmitted from symptomatic people to others who are in close contact through respiratory droplets, by direct contact with infected persons, or by contact with contaminated objects and surfaces. [5] In addition, repeated biological sampling from confirmed COVID-19 patients provide evidence that shedding of SARS-CoV-2 is highest in upper respiratory tract (nose and throat) acute (early) phase of the illness. [6] That is, within the first 3 days from onset of symptoms. [7] These set of people may be more contagious around this time of compared to later phase (convalescence) of the COVID-19. [3] This suggest the need for appropriate use of face masks by infected person.


Based on the transmission dynamics of SARS-CoV-2, the average time between exposure to the virus (becoming infected) and the onset of symptom (i.e. incubation period) for COVID-19 is on 5-6 days, however this can be can be up to 14 days. During this period, also known as the “pre-symptomatic” period, some infected persons can be contagious. Therefore, transmission via infectious droplets or touching contaminated surfaces from a pre-symptomatic case can occur before the onset of symptom. [3] This was evidenced from contact tracing efforts and enhanced investigation of clusters of confirmed pre-symptomatic cases. [8] This is supported by data suggesting that some people can test positive for COVID-19 from 1-3 days before they develop symptoms. [9] A person is said to be asymptomatic when confirmed SARS-CoV-2 positive by a molecular diagnostic test and do not develop any symptom before being tested negative twice at 24 hours interval. Worthy of note is that SARS-CoV-2 infected and asymptomatic persons can transmit the virus to previously uninfected persons.

Aside SARS-CoV-2 being transmitted mainly via respiratory droplets and contact with contaminated surfaces, a study has suggested the possibility of feco-oral transmission of SARS-CoV 2. [10] For instance, a study of 73 hospitalized COVID-19 patients showed 39 patients being positive for SARS-CoV-2 RNA in stool samples. [11] In addition, 17 patients remained positive for SARS-CoV-2 in stool even after becoming negative in respiratory samples. This plausible report might be due to the ability to demonstrate SARS-CoV-2 receptor, angiotensin converting enzyme 2 (ACE2) in gastrointestinal epithelial cells. Indeed, these findings have implications for possible SARS-CoV-2 fecal transmission and another reason to emphasize the need for good personal hygiene, such as frequent hand washing with soaps.  However, food borne acquisition of SARS-CoV-2 from contaminated foods has not been elucidated. Mother-to-child transmission has also been suspected but more studies needs to confirm vertical acquisition of SARS-CoV-2. [12]

An elaborated report from China had demonstrated sustained human-to-human transmission is the major means of acquiring SARS-CoV-2 among household contacts, where the secondary attack rate has been estimated to exceed 10%, in health care centers, and in overcrowded settings. However, widespread community transmission, as is currently being observed in many parts of the world including Nigeria requires more expansive transmission events between non-household contacts. [13] In some countries, community transmission SARS-CoV-2 had occurred in social or religious gathering and during funerals. Hence, community transmission is evidenced by the inability to relate confirmed cases through chains of transmission for a large number of cases, or by increasing positive tests through sentinel samples. [13]

In consideration of the recent report that demonstrated the viability SARS-CoV-2 in aerosol and fomite for 5-7 hours and days [14], respectively, it is very crucial that people be restricted from risks of contracting SARS-CoV-2.

The views expressed in this article are the author`s own and do not necessarily reflect the editorial views of Science Communication Hub Nigeria and/or the African Science Literacy Network (ASLN).

Idris Nasir Abdullahi

Lecturer and Specialist in Medical Virology

Ahmadu Bello University, Zaria, Nigeria

Email: inabdullahi@abu.edu.ng, eedris888@yahoo.com

 

References

  1. Worldometers.info. Situation Update Worldwide, as of 14th April, 2020. 2020; Dove, Delaware, USA; Retrieved from: https://www.worldometers.info/coronavirus/#countries Last accessed 7:00AM GMT+1, 14th April, 2020

  2. Wang D, Hu B, Hu C, Zhu F, Liu X, Zhang J, et al. Clinical Characteristics of 138 Hospitalized Patients With 2019 Novel Coronavirus–Infected Pneumonia in Wuhan, China. JAMA 2020; 323(11):1061-1069; doi:10.1001/jama.2020.1585.

  3. World Health Organization. Coronavirus disease 2019 (COVID-19) Situation Report – 73. Retrieved from https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200402-sitrep-73-covid-19.pdf?sfvrsn=5ae25bc7_2 Last accessed 13th April, 2020

  4. Soon-Shiong P. The Science Behind the Coronavirus (PowerPoint slides). 2020; Retrieved from https://youtu.be/ddlRvghGdPk. Last accessed 13th April, 2020

  5. Ong SW, Tan YK, Chia PY, Lee TH, Ng OT, Wong MS, et al. Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. JAMA. 2020 Mar 4

  6. Lauer SA, Grantz KH, Bi Q. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application. Ann Intern Med 2020 doi: 10.7326/M20-0504

  7. Liu Y, Yan LM, Wan L. Viral dynamics in mild and severe cases of CVOID-19. Lancet Infect Dis doi.org/10.1016/S1473-3099(20)30232-2

  8. Pan X, Chen D, Xia Y. Asymptomatic cases in a family cluster with SARS-CoV-2 infection. Lancet Infect Dis 2020 doi : 10.1016/ S1473-3099(20)30114-6

  9. Wei WE, Li Z, Chiew CJ, Yong SE, et al. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR, 1 April 2020/69

  10. Holshue ML, DeBolt C, Lindquist S, Lofy KH, Wiesman J, Bruce H, et al. First Case of 2019 Novel Coronavirus in the United States. N Engl J Med. 2020; 382:929-936. DOI: 10.1056/NEJMoa2001191.

  11. Xu Y, Li X, Zhu B. Characteristics of pediatric SARS-CoV-2 infection and potential evidence for persistent fecal viral shedding. Nat Med 2020; 26: 502–505 https://doi.org/10.1038/s41591-020-0817-4

  12. Zhu H, Wang L, Fang C, Peng S, Zhang L, Chang G, et al. Clinical analysis of 10 neonates born to mothers with 2019-nCoV pneumonia. Translational Pediatrics 2020; 9 (1): 51-60.

  13. Ghina I, Woods S, Ritger KA, McPherson TD, Black, SR Community Transmission of SARS-CoV-2 at Two Family Gatherings —Chicago, Illinois, February–March 2020. MMWR. 2020; Vol. 69

  14. Doremalen VN, Bushmaker T, Morris D, Holbrook M, Gamble A, Williamson B, et al. Aerosol and surface stability of HCoV-19 (SARS-CoV-2) compared to SARS-CoV-1. New Eng J Med. 2020; doi: 10.1056/NEJMc2004973.

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