The passage below is taken from a publication by Sanford et al titled « Information Loss: Potential for Accelerating Natural Genetic Attenuation of RNA Viruses »
RNA viruses are excellent candidates for genetic degeneration because they
typically have an extraordinarily high mutation rate [14]. The higher mutation rate
of RNA viruses is a consequence of the novel mechanisms required for RNA
replication, which are especially prone to mutation, and the lack of effective repair
enzymes for RNA replication. Even in RNA viruses with relatively small genomes,
there appear to be as many as 0.1 to 1.0 new mutations per virus per replication
cycle [15]. The mutation rate in RNA viruses is so high that it becomes difficult
to speak of a given viral “strain”, because any genotype quickly mutates into a
complex of genotypes, such that any patient is soon infected with a “viral swarm”.
With such a high mutation rate, the large majority of viral genotypes in a patient
must carry many deleterious mutations, and so will be inferior to the original
infecting genotype. This implies the lack of a realistic mechanism to preserve a
“standard genotype”, and all RNA viral swarms should typically be on the verge
of mutational meltdown.
When a virus is transmitted from one individual to the next, the first individual
harbors a viral swarm. The second individual becomes infected by a random
subset of that swarm (conceivably a single genotype). With this type of bottleneck-
ing, the “best” viral genotypes within the first swarm have a small probability of
being transmitted to the next host. This probability becomes especially small when
infection arises from a single viral particle. Given a high mutation rate and regular
bottlenecks, the operation of Muller’s Ratchet becomes quite certain, which
should result in a continuous ratchet-like mutational degeneration of the viral
genome [6].
It seems to me that the logic of this passage is unassailable and therefore the conclusion that RNA viruses cannot escape slow erosion/natural attenuation is inescapable. Do you see what is wrong in this reasoning ?
For the record, here is the whole paper:
https://www.worldscientific.com/doi/pdf/10.1142/9789814508728_0015