How and why does a virus mutate?

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The SARS-CoV-2 coronavirus is mutating, and currently two variants in particular are being closely watched around the world: The British (B.1.1.7) and South African (B.1.351) variants. They are said to be significantly more contagious, and concerns are repeatedly expressed that vaccines may no longer be effective against certain mutations. It is perfectly normal for viruses to mutate. We asked exactly why this happens.

How do viruses reproduce?
Before you can understand mutation, you first have to understand how viruses replicate. “A virus, unlike bacteria, is not able to reproduce on its own. To do so, it has to infect cells and misuses their equipment,” virologist Lukas Weseslindtner from MedUni Vienna explains to futurezone. To do this, the virus releases its genome into the body cell.

RNA viruses, however, do not find tools, so-called RNA polymerases, in the human cell to reproduce, but bring them along themselves. A section of their genome contains the information for this “copying machine,” Weseslindtner explains. This is then produced by the cell and the viral genome is replicated.

When does a virus mutate?
The process of RNA copying is error-prone, according to Weseslindtner. As a result, the RNA virus is always slightly altered and mutations occur. “These errors are incorporated randomly, because the virus is not intelligent and does not pursue a goal when mutating,” the virologist explains. The virus behaves according to Darwin’s laws of nature, according to which variants better adapted to the host prevail. Selection therefore takes place. Mutations therefore happen all the time, and many disappear again immediately if they harm the virus.

How does a mutation become a variant?
However, certain mutations now lead to biological changes in the virus that favor its spread. So evolution takes place and the virus gets better and better at infecting a new host. Viral strains with a fixed combination of mutations are called variants. Thus, if such a variant remains stable, it may be related to the fact that it replicates faster or is more infectious. This is most likely the case with B. 1.1.7 or B. 1.351. But a variant also keeps mutating.

Why can variants be dangerous?
Variants are usually more effective compared to existing virus strains. For example, they bind better to receptors in the human cell, which increases their infectivity, or they are more resistant to the host’s immune defenses, such as the antibodies trained by humans. This has now been demonstrated in the South African variant. Here, in addition to a higher infection rate, the mutation E484K was also detected, which causes poorer binding by antibodies possessed by recovered individuals. This could be reinfected more easily by the South African variant.

However, Weseslindtner said, evolution in SARS-CoV-2 is still occurring relatively slowly, especially compared to other RNA viruses we the influenza or hepatitis C virus. “The flu virus (influenza) can exchange entire gene segments with other influenza viruses. The hepatitis C virus constantly creates new variants within a host within a very short time.”