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Originally named  2019 Novel Coronavirus (2019-nCoV), now officially named Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is a positive-sense, single-stranded RNA coronavirus. The virus is the causative agent of coronavirus disease 2019 (COVID-19) and is contagious through human-to-human transmission. The virus is thought to have a zoonotic origin, and comparisons of the genetic sequences of this virus and other virus samples have shown similarities to SARS-CoV and bat coronaviruses, which makes an ultimate origin in bats likely. However, an intermediate reservoir such as a pangolin is thought to have been involved.

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Source and Suspected Origin

 

It is not yet known how the SARS-CoV-2 virus first entered humans. The initial suspect was the ant-eating pangolin, but recent evidence casts some doubt on this theory. Both SARS and MERS are thought to have originated in bats, which are also seen as a reservoir for coronaviruses, among other pathogens. Identifying the virus source can be important for preventing new outbreaks.

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Among humans, it can be spread through a variety of routes, including aerosolized droplets, contaminated surfaces, and possibly fecal-oral routes (the latter of which would increase spread in low-sanitation areas). The vast majority of transmission appears to be through similar routes as the cold and flu viruses (respiratory droplets and contaminated surfaces).

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How is Coronavirus Transmitted?

 

Coronavirus is spread from an infected person through:

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• Droplets spread when a person coughs or sneezes

• Close personal contact such as touching or shaking hands

• Touching an object or surface with the virus on it, then touching your mouth, nose or eyes before washing your hands

 

Structure and Properties

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Coronaviruses are large RNA viruses with spikes on them, vaguely resembling that of a  crown. Hence, the name CORONAvirus, as “corona” means crown in Latin. Strains of Coronaviruses are common throughout the animal world with varying effects ranging from mild to severe.

 

Alpha & Beta Coronavirus

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The two major categories of Coronavirus in humans are Alpha-Coronavirus and Beta-Coronavirus. SARS-CoV-2 is a Beta-Coronavirus, as are MERS and SARS. Compared to SARS, SARS-CoV-2 appears to be more transmissible, meaning having a higher average number of people infected by someone who is already infected.

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Transmissibility is captured by the basic reproduction number (R0), the number of people likely to catch an infection from someone who’s been infected themselves. If a virus has an R0 under one, it’s likely to die out. Estimates of the R0 for the Novel Coronavirus range from a median of 1.95 (given by the WHO early on) to a median of 2.28 or 2.79 (with a range of 1.6-4.2) suggested by other studies, depending on the time point and population (e.g. the passengers aboard the Diamond Princess cruise ship versus other populations not on a cruise ship). For comparison, seasonal flu has an R0 of 1.28, H1N1 influenza is at 1.4-1.6, the 1918 influenza pandemic was around 1.8, and SARS was in the range of 2-5.

An R0 greater than 1 doesn’t mean that a major epidemic is inevitable, or that the virus will spread uncontrolled. Interventions can limited transmission rates, such as disease surveillance, isolation of cases, closing schools, safety measures at airports, and in the future, possibly immunizations.

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Organ systems (Lungs)

 

The effects on lungs are highly variable, seeming to be more diverse than initially anticipated. On CT scans, “ground-glass” opacities are commonly seen in the lungs, referring to fluid buildup or thickening of tissue where air spaces should normally be. Additional patterns in the lungs that suggest pneumonia as a cause of tissue injury are also appearing. COVID-19 seems to have especially harmful impacts on the lower respiratory tract. That’s because the primary human receptor for the SARS S glycoprotein, called human angiotensin-converting enzyme 2 (ACE2), is mainly found in the lower respiratory tract. There appear to be three major manifestations of lung symptoms: mild illness with upper respiratory tract symptoms, pneumonia that isn’t life-threatening, and severe pneumonia with acute respiratory distress syndrome (ARDS). The latter course can start with mild symptoms for around a week and then progress rapidly to much difficulty breathing, requiring life support.

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Effects on the Immune System

 

3.1 Leukocytes

Patients with COVID-19 commonly have low levels of lymphocytes, which may be associated with mortality and symptom severity.

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3.2 Inflammatory Signalling

COVID-19, along with MERS and SARS, all showed exceptionally high levels of proinflammatory cytokines in serum, including IL1B, IFNγ, IP10, and MCP1. Cytokine storm, which is a massive overproduction of immune cells in response to infection, appears to be associated with the severity of COVID-19. Elevated CRP levels are common in patients admitted to hospitals for COVID-19.

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Effects on Mortality

 

The case fatality rate of COVID-19 is uncertain, and continually being updated with new evidence by international and national health agencies. It appears to be somewhere around 2%. MERS had a much higher rate (roughly around 35%),[ as did SARS (roughly around 10%).