Coronaviruses

Coronavirus

Morphology 1 / 2

Coronaviruses viewed under an electron microscope, with their halo, or crown-like (corona) appearance. From Medical News Today.

Morphology Phylogeny

Overview & taxonomy

Coronaviruses belong to the subfamily Orthocoronavirinae within the family Coronaviridae (order Nidovirales) ( phylogeny). They are enveloped, positive-sense single-stranded RNA (+ssRNA) viruses that infect a wide array of avian and mammalian hosts, including humans.

The subfamily Orthocoronavirinae is divided into four genera:

• Alphacoronavirus: Includes endemic human coronaviruses HCoV-229E and HCoV-NL63.
• Betacoronavirus: Includes HCoV-OC43, HCoV-HKU1, SARS-CoV-1, MERS-CoV, and SARS-CoV-2.
• Gammacoronavirus & Deltacoronavirus: Primarily infect birds and marine mammals.

Human coronaviruses were first identified in the 1960s from nasal cavities of patients with upper respiratory tract infections. Four endemic human coronaviruses (HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1) circulate continuously in human populations, accounting for 15% to 30% of common cold cases globally. Symptoms usually set in 2 to 4 days post-exposure and are generally mild (sneezing, rhinorrhea, fatigue, mild cough, and sore throat), though they can exacerbate underlying respiratory conditions such as asthma or COPD.

Virion morphology & genome architecture

• Morphology: Virions are pleomorphic or quasi-spherical, ranging from 80 to 160 nm in diameter. They are enveloped by a lipid bilayer studded with prominent, club-shaped Spike (S) glycoprotein trimers that give the virion a characteristic "halo" or solar corona appearance under cryo-electron microscopy.
• Structural proteins: The viral envelope contains three or four primary structural proteins:
1. Spike (S): Mediates attachment to host surface receptors and viral entry.
2. Envelope (E): Facilitates virion assembly, budding, and pathogenesis.
3. Membrane (M): Defines virion shape and drives membrane interaction.
4. Nucleocapsid (N): Encapsidates the RNA genome inside the virion
• Genome: Coronaviruses possess the largest genomes among all known RNA viruses, ranging from 26 to 32 kilobases (kb) in length. The genome features a 5'-cap and 3'-polyadenylated tail and functions directly as infectious mRNA upon host entry.
• Gene organization: Consistently conserved as 5'--ORF1ab (replicase/transcriptase)--S--E--M--N--3' , interspersed with lineage-specific accessory genes.

Highly pathogenic human coronaviruses of the 21st Century

While seasonal coronaviruses cause mild disease, three zoonotic betacoronaviruses have spilled over into human populations during the 21st century, causing severe acute respiratory illness:

1. SARS-CoV-1 (Severe Acute Respiratory Syndrome)

• Emergence: Emerged in Guangdong province, China, in late 2002 and spread globally in 2003, infecting ~8,098 individuals across 29 countries.
• Mortality: Case fatality rate (CFR) of ~9.6% (~774 deaths), rising above 50% in individuals over 65 years old.
• Origin & pathogenesis: Originates in horseshoe bats (Rhinolophus spp.), with civet cats serving as the primary intermediate host. SARS-CoV-1 utilizes angiotensin-converting enzyme 2 (ACE2) as its cellular entry receptor. The virus was contained by July 2003 through strict public health measures.

2. MERS-CoV (Middle East Respiratory Syndrome)

• Emergence: First identified in Saudi Arabia in September 2012.
• Mortality: Case fatality rate of ~34% to 36% (over 880 deaths out of ~2,600 cases), marking it as the most lethal human coronavirus.
• Origin & pathogenesis: Reservoir hosts are bats, but dromedary camels serve as the primary direct source of zoonotic transmission to humans. MERS-CoV uses dipeptidyl peptidase 4 (DPP4 / CD26) as its host receptor and causes severe pneumonia, acute respiratory distress syndrome (ARDS), and acute renal failure. Interhuman transmission occurs primarily in healthcare settings with close contact (WHO,2025).

3. SARS-CoV-2 (Coronavirus disease 2019 / COVID-19)

• Emergence: Emerged in Wuhan, Hubei Province, China, in December 2019, triggering a global pandemic declared by the World Health Organization (WHO) in March 2020.
• Mortality & impact: Over 770 million confirmed cases and over 7 million recorded deaths worldwide. While its overall infection fatality rate (IFR) is lower than SARS-CoV-1 and MERS-CoV (~0.5%–1.5% overall, varying widely by age and vaccination status), its extremely high transmissibility resulted in vastly higher absolute disease burden and mortality.
• Origin & pathogenesis: Shares ~79% genome sequence identity with SARS-CoV-1 and ~96% identity with bat coronavirus RaTG13. Like SARS-CoV-1, SARS-CoV-2 binds to ACE2 via its Spike protein, augmented by a novel furin cleavage site that enhances tissue tropism and transmissibility. Rapid global adaptation led to sequential variants of concern (Alpha, Beta, Gamma, Delta, Omicron and its sublineages).