2.4 Virus evolution
What drives the evolution of a virus? Different strains of a virus are subject to two main types of selective pressure. In a susceptible host population, a strain that can spread more quickly and/or reproduce more quickly and efficiently has a selective advantage over other strains. The second pressure on a virus is created by immunity in the host population, which may come from infection or vaccination.
With SARS-CoV2, we have seen both of these selective pressures acting on the virus. The increase in R0 values in later variants indicates that the virus progressively adapted to the human population after it had initially jumped species from bats. The independent appearance of new variants that could evade the immune response against earlier variants, is evidence of the selective pressure produced by immunity in individuals and the population as a whole.
It is worth noting here that a virus cannot mutate indiscriminately and still retain its ability to bind to target cells and replicate. Some viral structural components and enzymes must be retained. Immune responses (antibodies or T cells) recognising these less-variable viral components are likely to confer some immunity that acts across different strains.
In the last part of the course, we will look in more detail as to how variants have partly evaded immunity produced by previous infections and how vaccines can be modified to provide continued protection against infection and disease.