The mRNA in an mRNA vaccine is modified to prevent degradation in vivo, and this could in theory cause some differences versus exogenous proteins when translated in vivo*. The LNP as you know which protects the mRNA also serves as an adjuvant, and perhaps this can affect durability versus other adjuvants (or even non adjuvenated vaccines like PFE's RSV vaccine)
*There is some data showing more errors in translation versus native RNA, so perhaps antibodies to these mutated proteins can interfere with immunity to the intended protein longer term??
just spitballing here
Edit: Let me just also add that biology is inherently complex, as you know, and whenever you are using a new tech there may always be some unintended consequences that you cannot predict simply mechanistically so you don't have to have some rationale as to why durability could be affected to have an effect on durability. The data is the data, and now after the RSV data in particular the onus (rightly or wrongly) may be on MRNA to show that their vaccines produce lasting immunity when they go up against a vaccine like shingrix that confers more than a decade of protection.
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