The key features of the mRNA vaccines are the use of modified nucleotides in their synthesis and the use of lipid nanoparticles to deliver them to cells. The main difference between the Pfizer/BioNTech vaccine and the Moderna vaccine is in the delivery system. The lipid vescicules used by Moderna are somewhat more stable and the vaccine doesn't need to be kept constantly at ultra-low temperatures.
Both vaccines use modified RNAs. They synthesize the RNA using modified nucleotides based on variants of uridine; namely, pseudouridine, N1-methylpseudouridine and 5-methylcytidine. (The structures of the nucleosides are from Andries et al., 2015).) The best versions are those that use both 5-methylcytidine and N1-methylpseudouridine.
I'm not an expert on these mRNAs and their delivery systems but the way I understand it is that regular RNA is antigenic—it induces antibodies against it, presumably when it is accidently released from the lipid vesicles outside of the cell. The modified versions are much less antigenic. As an added bonus, the modified RNA is more stable and more efficiently translated.
Two of the key papers are ...
Andries, O., Mc Cafferty, S., De Smedt, S.C., Weiss, R., Sanders, N.N. and Kitada, T. (2015) "N1-methylpseudouridine-incorporated mRNA outperforms pseudouridine-incorporated mRNA by providing enhanced protein expression and reduced immunogenicity in mammalian cell lines and mice." Journal of Controlled Release 217: 337-344. [doi: 10.1016/j.jconrel.2015.08.051]
Pardi, N., Tuyishime, S., Muramatsu, H., Kariko, K., Mui, B.L., Tam, Y.K., Madden, T.D., Hope, M.J. and Weissman, D. (2015) "Expression kinetics of nucleoside-modified mRNA delivered in lipid nanoparticles to mice by various routes." Journal of Controlled Release 217: 345-351. [doi: 10.1016/j.jconrel.2015.08.007]
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Prophylactic immunization requires more than injecting a pathogen surface protein. For long it was the immunologists' "dirty little secret" that an adjuvant was required. Thus, the rebirth of studies on innate immunity.
Happily, the SARS-CoV-2 mRNA vaccine achieves this by bringing its own sort-of adjuvent. With a lipid coating it resembles the actual virus (plus strand RNA), without all the troublesome trimmings. Once inside a host cell, the single stranded RNA will either be recognized by Toll receptors, or, perhaps better still, will form a sufficient segment of dsRNA to activate the interferon stress response.
Marcus in 1983 reported that one molecule of RNA, should it find a complementary partner, would suffice to trigger an interferon response (Interferon 5, 115-180). So, say, a coronavirus mRNA "vaccine" is injected and one gets in a host cell that happens to have a complement (perhaps aka "junk RNA"). The dsRNA, courtesy of interferon, enhances MHC protein expression, while the mRNA then shakes itself loose and gets translated. The resulting foreign peptides form pMHC complexes that are displayed at the cell surface just in time to entice a passing T-cell. The rest is history!
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