A variant is a virus that shows genetic changes compared to an “original” version of the virus. In the case of COVID-19, the original version is the one that appeared in China in late 2019. The SARS-CoV-2 chromosome, which contains the genetic material, can be described as a word 30,000 letters long consisting of the four letters A, U, G and C. The exact order of these letters determines the structure of the 15 genes responsible for making the virus proteins. Any change, however small, in this long sequence of letters is likely to result in changes to one or more of these proteins.
When the virus replicates, a copy of the original chromosome is incorporated into the new virus particles, but each copy may contain one or more errors that distinguish it from the original version. These differences are called mutations. Some of these mutations will be “lethal” and prevent the formation of new virus particles. Other mutations will be passed on to entire generations of new viruses, which are called “variants.”
The vast majority of variants behave exactly the same as the original virus and are therefore of no clinical interest. Others, such as the British, South African, Brazilian and Indian variants, are of greater concern because they modify the famous S (spike) protein, which is largely responsible for the infectivity of SARS-CoV-2. The problem is that the vaccines are based on the original S protein (without mutations) and may not protect us against these variants with different S proteins.
To detect each new mutation of the virus, the order of the 30,000 letters that make up the virus chromosome must be determined and compared to the order of the original chromosome. This is called sequencing. It allows us to identify any new variants.
Screening, on the other hand, allows known variants to be identified more quickly by analyzing only the portion of the chromosome that contains the mutation.
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