Still watching the emerging lineages of SARS-CoV2 I noticed that the amino acid mutation from threonine to isoleucine seems to be particularly frequent. Counting mutations in a lineage with a lot of mutation (I choose BA.4) kind of confirmed my anecdotal impression: Indeed, T-to-I mutations were the most frequent ones.
They are more frequent than alanine to valine mutations, and the difference in frequency is not explained by a difference in the frequency of the amino acids in the virus proteines (Threonine is somewhat more frequent than alanine, but not that much).
In part this phenomenon can be explained that a T-to-I mutation involves a nucleotide change from C to T which is the most frequent nucleotide mutation of Coronaviruses in humans. However, in order to be fixated in a lineage, the T-to-I mutation needs also to provide some evolutionary advantage. It seems to be a generic advantage of that kind of mutation.
So my question is: What are evolutionary advantages of T-to-I mutations of SARS-CoV2 in humans?