What is the motivation behind the CCA -> CCU modification between SARS-CoV-2 and the mRNA vaccine when encoding the signal peptide?
The other answer is a bit convoluted IMHO.
First note that as the article cited notes, that's not actually CCA -&gt; CCU but CCA -&gt;  CCΨ; they only used ASCII for convenience. Furthermore, these three all code for proline as the article also notes.
Second, Ψ stands for pseudouridine and it's the golden discovery of Kariko and colleagues (which made mRNA vaccines [more] practical) that you need to use as many pseudo-Us as possible to ensure that the exogenous mRNA doesn't get destroyed prematurely and/or triggers downregulation of transcription.
Third, that's not even pseudo-U that's actually used in the Pfizer vaccine but N1-methyl-pseudouridine (variously abbreviated as N1mΨ or m1Ψ), which is even more &quot;potent&quot; in these terms than even pseduo-U. (According to a book on RNA therapeutics, m1Ψ was actually discovered and is patented by Moderna circa 2012-2013, before any academic publications.) Actually a 2017 publication found that independently of reduced immunogenicity, m1Ψ seems to be more active &quot;by increasing ribosome pausing and density on the mRNA&quot;:

We show that in addition to turning off the immune/eIF2α phosphorylation-dependent inhibition of translation, the incorporated N1mΨ nucleotides dramatically alter the dynamics of the translation process by increasing ribosome pausing and density on the mRNA. Our results indicate that the increased ribosome loading of modified mRNAs renders them more permissive for initiation by favoring either ribosome recycling on the same mRNA or de novo ribosome recruitment.

And more research on the latter aspect (2019)

Global incorporation of different modified nucleotides often (but not always) markedly changed mRNA expression. This effect was seen across numerous synonymous coding variants of multiple proteins, in several different cell lines, and in vivo. m1Ψ generally gave higher expression than U or mo5U for the same sequence. Biophysical studies revealed that m1Ψ and mo5U have dramatically different and opposite effects compared to U (stabilizing and destabilizing, respectively) on overall mRNA folding, nearest-neighbor base-pairing thermodynamics, and secondary structure pattern as mapped by SHAPE.

There could be additional reasons, but these are probably marginal.
Now, canadianer asks an astute question below (which was not asked by the OP), whether it's (still) preferable to do UUU -&gt; UUC (which normally it is) if m1Ψ is going to be globally substituted for U anyway. I don't actually know the answer to that; Pfizer or Moderna might have researched it, but I haven't found any publication on the trade-off.
The actual reason for the latter (especially since UUC is about 3 times as inefficient as UUU) might be that real goal is to slow down the protein folding there. The WHO doc does say that the protein is folded in a particular way &quot;to ensure the S glycoprotein remains in an antigenically optimal pre-fusion conformation&quot;. In general, there's a trade-off between the speed of translation and accuracy of folding. (ΨUU is actually twice as efficient as UUU already, even with naturally occurring pseudouridine. In fact UUU is one of the most common places where one finds a naturally occurring pseudouridine.)