Etiology

• SARS-CoV-2 is an enveloped, nonsegmented, positive-sense, ssRNA β-coronavirus.

• The viral genome encodes:

  • Four structural proteins

    • Spike: facilitates entry by attaching to the ACE2 receptor of the host cell

    • Envelope

    • Membrane

    • Nucleocapsid: packages the viral genome into a helical ribonucleocapsid

  • 16 nonstructural proteins: form the replicase–transcriptase complex

Increased transmissibility

• Decreased severity [12]

• Decreased neutralization by antibodies [

• The basic reproduction number (R0) of the original strain is estimated to be ∼ 2–3.

• The R0 is higher in newer variants compared to the original strain.

The primary mode of transmission is exposure to respiratory fluids via either:

• Inhalation of droplets or aerosol particles

  • The concentration of aerosol particles is highest within 3–6 feet of the infectious source.

  • Small aerosol particles can remain suspended in the air for minutes to hours.

  • The risk of transmission is increased in poorly ventilated areas.

• Mucous membrane contact with respiratory particles

1. The spike protein binds to the membrane protein angiotensin-converting enzyme 2 (ACE2). [28]

2. Transmembrane protease serine 2 (TMPRSS2) activates the spike protein.

3. Membrane fusion and uncoating of the viral RNA occur.

• Activation of the immune system → release of cytokines (e.g., tumor necrosis factor, IL-1β, IL-6) → acute inflammatory response

• Some patients demonstrate a very large immune response resembling a “cytokine storm” that can lead to organ failure and death.