COVID-19 infection: Origin, transmission, and characteristics of human coronaviruses

Abstract

The coronavirus disease 19 (COVID-19) is a highly transmittable and pathogenic viral infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which emerged in Wuhan, China and spread around the world. Genomic analysis revealed that SARS-CoV-2 is phylogenetically related to severe acute respiratory syndrome-like (SARS-like) bat viruses, therefore bats could be the possible primary reservoir. The intermediate source of origin and transfer to humans is not known, however, the rapid human to human transfer has been confirmed widely. There is no clinically approved antiviral drug or vaccine available to be used against COVID-19. However, few broad-spectrum antiviral drugs have been evaluated against COVID-19 in clinical trials, resulted in clinical recovery. In the current review, we summarize and comparatively analyze the emergence and pathogenicity of COVID-19 infection and previous human coronaviruses severe acute respiratory syndrome coronavirus (SARS-CoV) and middle east respiratory syndrome coronavirus (MERS-CoV). We also discuss the approaches for developing effective vaccines and therapeutic combinations to cope with this viral outbreak.

Keywords: COVID-19; Coronaviruses; Origin; Outbreak; Spread.

Graphical abstract

Fig. 1

Structure of respiratory syndrome causing human coronavirus.

Fig. 2

The key reservoirs and mode of transmission of coronaviruses (suspected reservoirs of SARS-CoV-2 are red encircled); only α and β coronaviruses have the ability to infect humans, the consumption of infected animal as a source of food is the major cause of animal to human transmission of the virus and due to close contact with an infected person, the virus is further transmitted to healthy persons. Dotted black arrow shows the possibility of viral transfer from bat whereas the solid black arrow represent the confirmed transfer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

The life cycle of SARS-CoV-2 in host cells; begins its life cycle when S protein binds to the cellular receptor ACE2. After receptor binding, the conformation change in the S protein facilitates viral envelope fusion with the cell membrane through the endosomal pathway. Then SARS-CoV-2 releases RNA into the host cell. Genome RNA is translated into viral replicase polyproteins pp1a and 1ab, which are then cleaved into small products by viral proteinases. The polymerase produces a series of subgenomic mRNAs by discontinuous transcription and finally translated into relevant viral proteins. Viral proteins and genome RNA are subsequently assembled into virions in the ER and Golgi and then transported via vesicles and released out of the cell. ACE2, angiotensin-converting enzyme 2; ER, endoplasmic reticulum; ERGIC, ER–Golgi intermediate compartment.

Fig. 4

Betacoronaviruses genome organization; The Betacoronavirus for human (SARS-CoV-2, SARS-CoV and MERS-CoV) genome comprises of the 5′-untranslated region (5′-UTR), open reading frame (orf) 1a/b (green box) encoding non-structural proteins (nsp) for replication, structural proteins including spike (blue box), envelop (maroon box), membrane (pink box), and nucleocapsid (cyan box) proteins, accessory proteins (light gray boxes) such as orf 3, 6, 7a, 7b, 8 and 9b in the SARS-CoV-2 genome, and the 3′-untranslated region (3′-UTR). The doted underlined in red are the protein which shows key variation between SARS-CoV-2 and SARS-CoV. The length of nsps and orfs are not drawn in scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Phylogenetic tree of coronaviruses (content in red is the latest addition of newly emerged SARS-CoV-2 and WSFMP Wuhan-Hu-1 is used as a reference in the tree); The phylogenetic tree showing the relationship of Wuhan-Hu-1 (denoted as red) to selected coronavirus is based on nucleotide sequences of the complete genome. The viruses are grouped into four genera (prototype shown): Alphacoronavirus (sky blue), Betacoronavirus (pink), Gammacoronavirus (green) and Deltacoronavirus (light blue). Subgroup clusters are labeled as 1a and 1b for the Alphacoronavirus and 2a, 2b, 2c, and 2d for the Betacoronavirus. This tree is based on the published trees of Coronavirinae , and reconstructed with sequences of the complete RNA- dependent RNA polymerase- coding region of the representative novel coronaviruses (maximum likelihood method using MEGA 7.2 software). severe acute respiratory syndrome coronavirus (SARS- CoV); SARS- related coronavirus (SARSr- CoV); the Middle East respiratory syndrome coronavirus (MERS- CoV); porcine enteric diarrhea virus (PEDV); Wuhan seafood market pneumonia (Wuhan-Hu-1). Bat CoV RaTG13 Showed high sequence identity to SARS-CoV-2 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)