Why is the SARS-CoV-2 virus so contagious?

coronavirus spread
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The coronavirus SARS-CoV-2 that causes coronavirus disease (COVID-19) is causing a global health crisis on the scale that has not been seen for over a century.

A coronavirus is part of a family of coronaviruses, which may make either animals or humans ill. Regarding humans, several coronaviruses are known to cause respiratory infections - ranging from a common cold, to more serious diseases such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS).

The most recently discovered coronavirus causes coronavirus disease COVID-19. It is understood that the most concerning aspect of COVID-19 is it’s apparent ability to spread faster than the other known associated coronaviruses.

According to the World Health Organisation (WHO), “The disease can spread from person to person through small droplets from the nose or mouth which are spread when a person with COVID-19 coughs or exhales. These droplets land on objects and surfaces around the person. Other people then catch COVID-19 by touching these objects or surfaces, then touching their eyes, nose or mouth. People can also catch COVID-19 if they breathe in droplets from a person with COVID-19 who coughs out or exhales droplets.”

What makes the new coronavirus so dangerous to humans is simply that it’s novel - meaning it’s new to humans, so we don’t have any way to fight it.

In recent weeks, research teams around the world have been working to uncover the molecular mechanisms that explain why this virus has such a high transmission capacity and understand exactly how it enters into human tissues.

"Understanding the transmission of the virus is key to its containment and future prevention," says David Veesler, a virology researcher at the University of Washington in Seattle. 

The new coronavirus spreads significantly faster than the one that caused severe acute respiratory syndrome, or SARS, and has infected more than ten times the number of people who contracted SARS.

A handful of genetic and structural analyses have identified a key feature of the virus — a protein on its surface — that might explain why it infects human cells so easily.

In late February, Veesler’s research group published the results of their SARS-CoV-2 analysis, which was shared on the bioRxiv biomedical server. 

Coronavirus: the spiky invader

To infect a human host, viruses must be able to gain entry into individual human cells.

Researchers at the University of Texas described a tiny molecular key on SARS-CoV-2 that gives the virus entry into the cell. This key is called a ‘spike’ protein (or S-protein) that binds to the cell membrane, a process that's activated by specific cell enzymes. All known coronaviruses are understood to enter cells in this manner.

José Aguilar Gavilán, researcher and professor of virology at the University of Córdoba explains: "To infect, viruses always use something that is essential for the cell, something that cannot mutate to defend itself.”

Genomic analyses of the new coronavirus COVID-19 have established that its particular spike protein differs from those of close relatives such as MERS and SARS, and suggests that the protein has an activation site that is  stimulated by a host-cell enzyme called furin.

This is an important finding, because furin is found in numerous human tissues, including the lungs, liver and small intestines, which means that the virus has the potential to attack multiple organs, explains Li Hua, a structural biologist at Huazhong University of Science and Technology in Wuhan, China, where the outbreak began.

José Aguilar Gavilán explains that because “the enzyme that SARS-CoV-2 is able to recognise, and that others cannot, is present in more tissues and more cells - this means that the virus, apart from infecting more quickly, has the ability to penetrate through different areas, not just the respiratory areas. It could even potentially be transmitted by other routes."

The furin enzyme link could explain some of the symptoms seen in patients with coronavirus - such as liver failure.

Li Hua’s team is looking at molecules that could block furin, which could be investigated as possible treatments or therapies. But their progress has been slow as a result of the impact of the outbreak.

Other research teams are demonstrating how the COVID-19 virus spike protein binds with a “high affinity” to certain cell receptors. This suggests that the specific receptor could also be a potential target for vaccines or therapies. 

Some researchers are cautious about overstating the role of the activation site in helping the coronavirus to spread more easily. Other scientists are also wary of comparing furin activation sites on flu viruses to those on the new coronavirus. 

Developing either drugs or a vaccine will be a challenging task. Treatments and vaccines not only have to evidence their effective defence against the virus, but must also be tested to ensure they are safe for people.

Centers for Disease Control and Prevention (USA) officials have said that the earliest a coronavirus vaccine could be available is in a year to a year and a half.

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