According to the WHO submission and one of the preprints from BioNTech/Pfizer, the 3'UTR of their Covid-19 vaccine's mRNA is the combo of the AES and mtRNR1 sequences, which (in the preprint) BioNTech cites to a paper on library screening of 3'UTRs that achieve high mRNA stability etc.
But as far as as I can tell, BioNTech added a "CUC GAG" between the (doubled) stop codon and the beginning of the AES as it's given in Supplementary Table 3 of the library paper cited, i.e. the actual AES starts with CUG GUA CUG, or more precisely using the global Ψ substitution as given in the WHO submission, the sequence starting with the stop codon(s) is:
ΨGA ΨGA CΨC GAG CΨG GΨA CΨG CAΨ GCA
So, what's the point of the bolded "glue" CUC GAG?