Coronavirus (continued)
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Background
Observations of coronavirus infections were first reported in
the United States in 1931. Infected chickens on a North Dakota
farm were listless and gasping for air, resulting in high mortal-
ity. Infected embryos were born dwarfed, and their joints were
fused. It was determined that the "gasping disease"—infectious
bronchitis—was caused by a filterable agent, that is, infectious
bronchitis virus (IBV). During the 1960s, the first human coro-
naviruses were isolated from the nasal washings of a child who
had the typical signs and symptoms of a common cold.
Prior to the SARS pandemic in 2003, human coronavirus
229E (HCoV 229E) and HCoV OC43 were the only known coro-
navirus strains circulating in the human population, causing
15–29% of all common colds. Research indicates that more
than 30% of children test positive for antibodies (evidence
of infection) with either coronavirus strain within the first 12
months of life.
Coronavirus structure
The name coronavirus is derived from the Latin word corona
because its spike (S) proteins (Fig. 2), which protrude outside
of the virion (the complete, mature virus particle), resemble a
royal crown or the Sun's corona when viewed using an electron
microscope (Fig. 1). Coronaviruses are enveloped and contain a
single-stranded ribonucleic acid (RNA) genome. The S protein is
responsible for the attachment of the virus to a receptor pres-
ent on the surface of a host cell, and the protein mediates the
fusion of the coronavirus envelope with cellular membranes,
allowing entry into the host cell. In addition, the S protein is the
main target for antibodies produced by the body as an immune
defense against the coronavirus. The envelope is associated
with two additional transmembrane proteins: a small envelope
(E) protein and a membrane (M) protein (Fig. 2). Some coro-
naviruses contain an additional envelope protein, which is a
hemagglutinin-esterase (HE) spike protein.
Coronavirus genomes range in length from 26 to 32 kilobas-
es. These genomes are the longest in size of all viruses contain-
ing RNA genomes. Similar to most eukaryotic messenger RNAs,
coronavirus genomic RNA contains a 5'-end cap structure and a
polyadenylated 3'-end. Table 1 and Table 2 list various charac-
teristics of the coronaviruses that infect humans.
Fig. 2: Illustration of SARS-CoV-2. Note the spike (S) proteins protruding from the surface of the coronavirus, which impart
the look of the Sun's corona or crown. S proteins bind to host cell receptors for entry or infection of the cell. (Credit: Alissa
Eckert and Dan Higgins/Centers for Disease Control and Prevention)
