Week 8: Variants and immunity
1 Variants of SARS-CoV2
At the start of the pandemic, there was much debate as to whether the original SARS-CoV2 virus, first identified in Wuhan, would mutate to produce new strains. Some viruses, (eg measles) are remarkably stable; the measles vaccine is 97% effective and has not required any substantial modification for many years. In contrast, other viruses such as influenza-A mutate often and it is necessary to produce new vaccines for the new strains each year.
Viruses with an RNA genome tend to mutate more rapidly than those with a DNA genome, and those with a segmented genome such as influenza-A, can also change radically by a process called recombination. Since coronaviruses have an RNA genome, we might therefore have reasonably expected some new variants of SARS-CoV2 to arise.
It turned out that SARS-CoV2 did mutate extensively, but only some of the variants produced were sufficiently advantaged compared with earlier strains, that they were able to spread in the community. Notice that here we are considering changes that could be advantageous for the virus. Viruses evolve in much the same way as other ‘life-forms’ and a strain with a genetic advantage will eventually replace earlier strains.
ITQ _unit9.1.1
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Suggest three factors which could give a mutated virus an advantage over an earlier strain.
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The new strain might infect cells more effectively. It might go through its replication cycle more quickly. The assembly of the virus in infected cells or its release could be more efficient. The new strain might be able to evade the antibodies produced by an older strain.
Notice that production of disease was not included in this list. A virus that produces an asymptomatic infection might actually have some advantage, because infected people continue to circulate unknowingly in the community, thereby promoting virus spread. However the production of some symptoms such as coughing or sneezing may be advantageous for a respiratory virus, because they promote production of aerosol droplets that transmit the virus.
It is not simple. As you read through the following sections on new variants and virus evolution, think in terms of what is advantageous for the virus, rather than its human hosts.