A narrative review on the basic and clinical aspects of the novel SARS-CoV-2, the etiologic agent of COVID-19

Abstract

The novel SARS-CoV-2 is responsible for causing the ongoing outbreak of coronavirus disease 19 (COVID-19), a systemic infection in humans. Ever since it was first detected in December 2019, the number of confirmed cases has continued to increase. Within a short period, this disease has become a global issue, and therefore it is characterized as a pandemic. The current understanding and explanations are based on epidemiological, clinical and physiological observations. Besides, it remains a great challenge, as much remains to be understood about this new disease-causing virus. Therefore, we seek to provide an overview of SARS-CoV-2, including its classification, origin, genomic structure, replication cycle, transmission, pathogenesis, clinical aspects, diagnosis, treatments, prevention and vaccine options. We conducted a literature search for the articles published up to August 2020 using the keywords 'SAR-CoV-2' and 'COVID19' in medical databases; PubMed, google scholar, EMBASE, and web of science. Based on the information collected, the emerging COVID-19, caused by SARS-CoV-2, exhibits strong infectivity but less virulence in terms of severity of disease and mortality rates in certain age groups. It inflicts more damage in terms of peoples' health and well-being, social life, and global economic impacts. Unfortunately, there is no adequate global and standard response to this pandemic to date, and each country is facing a crisis based on its situation, expertise, and hypotheses. While there is no effective therapy and vaccine against the novel SARS-CoV-2 yet, preventive measures are the only tool available to our disposal to control the spread of the COVID-19 pandemic. Ongoing and future research is focused more on developing standard treatment strategies, and efficacious vaccines, which would be useful to tackle this pandemic globally.

Keywords: Replication; SARS-CoV-2; coronavirus disease 19 (COVID-19); pathogenesis; transmission.

Figure 1

Schematic representation of the type of Coronaviruses. HCoVs belong to either the Alphacoronaviruses or Betacoronaviruses. HCoV229E, human coronavirus 229E, HCoVOC43, human coronavirus OC43; HCoVNL63, human coronavirus NL63; HCoVHKU1, human coronavirus HKU1; SARS-COV, severe acute respiratory syndrome coronavirus; MERS-CoV, Middle East respiratory syndrome-related coronavirus; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 2

Schematic representation of the zoonotic transmission of SARS-CoV-2: the novel SARS-CoV-2 is believed to have originated from bats, then passed through an intermediate host possibly pangolin, to an individual (across the species barrier into humans) which then resulted in the human to human transmission within populations. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 3

Virion and genomic structure of SARS-CoV-2. (A) Schematic structure of the SARS-CoV-2 virion, (B) structural organization of the SARS-CoV-2 genome. Structural proteins, Spike (S), envelope (E), membrane (M) and nucleocapsid (N) proteins, non-structural proteins translated from ORF 1a and ORF 1b and accessory proteins, including 3a, 3b, 6, 7a, 7b, 8a, 8b, and 9b (for SARS-CoV), and 3a, 6, 7a, 7b, 8, and 10 (for SARS-CoV-2) are indicated. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; ORF, open reading frame.

Figure 4

The life cycle of coronavirus in host cells; the virion lifecycle begins when the spike (S) protein binds to ACE2, the cellular receptor. Then the virus releases its plus sense genomic RNA into the host cell. The minus strand is then formed from the plus strand genomic RNA, from which subgenomic mRNAs are formed by transcription and then translated into relevant viral proteins. Genome RNA and viral protein are assembled and then released out of the cell. SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; ER, endoplasmic recticulum; ACE2 receptor, angiotensin-converting enzyme 2 receptor.