Long before we had the printed word, text was copied by hand. Each word was written and re-written many times. And errors occurred.
Traditionally, in European culture, the process of copying words and generating new or multiple copies of a manuscript was a task taken up by monks. It is tedious work. And the monks would occasionally amuse themselves by including marginalia. But they also ended up inventing new words by miscopying. As a consequence, careful reading of ancient manuscripts can often be revealing of the evolution of language.
Mistakes in copying DNA and RNA can also happen and instead of leading to new language, each mistake leads to a mutation. Some are neutral and do not affect the viability of the resulting organism. Some reduce the fitness of the organism, leading to its death and culling from the population. And some provide a competitive advantage for the resulting organism.
A mutation or multiple mutations in an organism lead to changes in its viability and it’s upon these changes natural selection acts.
In the case of viruses, mutations arise as a natural by-product of viral replication. For example, a single letter change in the nucleic acid code can result in the substitution of a glycine for an aspartic acid in the resulting protein. That substitution may or may not make the protein better or worse at doing its job.
Because RNA is single stranded, RNA viruses typically have a higher mutation rate than DNA-based viruses where the second strand provides a form of error checking. In the sub-class “RNA-based coronaviruses,” however, the number of mutations is lower than for other RNA viruses because they encode an enzyme that corrects some of the errors made during replication. In effect, they have an intrinsic proofreader built into their genetic code.
But changes do occur allowing viruses to mutate and develop into different variants. And variants might have a single mutation or many. Strictly speaking, a variant becomes a strain of the virus when it has demonstrably different properties or phenotypes. That is, it is a new strain because it might be more transmissible or more virulent or even less fatal.
Variants succeed when they confer some form of evolutionary advantage to the virus. The variants which have emerged in Denmark, the United Kingdom, and South Africa result in changes to the structure of the spike proteins covering the surface of the virus. And as the spike protein is the target for the vaccines, these organisms become species of interest or concern.
In this case, “interest” means a “variant with specific genetic markers that have been associated with changes to receptor binding, reduced neutralization by antibodies generated against previous infection or vaccination, reduced efficacy of treatments, potential diagnostic impact, or predicted increase in transmissibility or disease severity,” according to the CDC. In other words, things which might make a virus more effective or deadly.
The term “concern”, as defined by the CDC, has pretty much the same meaning but adds “evidence of increase in transmissibility” and “diagnostic detection failures.” A variant of concern is a more deadly version of the virus which is more readily spread throughout the population and may not be observed in the tests used for detection.
The B.1.1.7 virus, for example, had accumulated 17 different mutations prior to its detection in September 2020. At the time, it was referred to as a “variant of interest” but by December it had become a “variant of concern” as it accounted for roughly 28 per cent of the cases of infection in England. It is spreading much more quickly than other lineages.
Further, and unlike some of the other variants which have been detected, it has achieved dominance by outcompeting the existing population of circulating variants. In evolutionary terms, it is “more fit” or a better strain of the virus. Not good for the human population but great for the SARS-CoV-2 community!
Eight of the mutations in B.1.1.7 are in the spike glycoproteins, which could possibly influence the ACE2 binding capacity of the virus and lead to increased viral replication. While the strain does not appear to be massively more fatal, an increase in mortality rates has been detected. And while the mutations in the spike glycoproteins do not appear to be sufficient to render them invisible to the antibodies generated by the vaccines, there is always the possibility the continual evolution of the virus may make the present versions of the vaccines obsolete.
From a public health perspective, what does all of this science mean? Simply that we need to keep doing what we have been doing. Measures like masks, physical distancing, and limitations on large gatherings need to remain in effect until the more transmissible variants are under control. Given the power of evolution, we may be in this for the long haul.
