How and why are mRNAs modified for the vaccines?

Given that mRNAs are naturally unstable, the manufacturers modify them for the vaccines. For instance, they modify individual components or the ends of the mRNAs. This is done for various reasons: Certain modifications increase the halflife of the mRNA. This is important to ensure that the mRNA can continue to supply host cells with the instructions necessary to produce sufficient numbers of spike proteins.

Another reason is that unmodified mRNA can also trigger a rapid, but unhelpful, short-lived and non-specific immune response. In contrast, the modified mRNA stimulate the immune system in a targeted manner that enables long-lasting protection against the coronavirus. Other modifications help the cells to produce greater amounts of protein.