On June 16, 2021, CureVac announced that its COVID-19 vaccine candidate had shown only 47% efficacy in a Phase III interim analysis—below the ≥50% threshold for approval. This was a significant setback, raising questions among researchers.

While CureVac attributed the result to the rapidly evolving virus, some researchers suspected the vaccine design. Dr. Drew Weissman of the University of Pennsylvania noted that the vaccine used unmodified mRNA—which, when injected, triggers interferon production. While interferons signal immune activation, they can also suppress T helper cell production, which in turn guides B cells to produce antibodies.

This case highlights the critical importance of molecular design in mRNA drug development to address stability, translation efficiency, and immunogenicity—ensuring safety and efficacy. These goals are achieved through various chemical modifications: 5' capping, 3' poly(A) tailing, and nucleotide modification.

Mature eukaryotic mRNA contains several key structural elements: 5' cap, 5' UTR, open reading frame (ORF) , 3' UTR, and poly(A) tail. In vitro-synthesized mRNA mimics natural mRNA structure.

The 5' cap (7-methylguanosine, m7G) regulates mRNA maturation, facilitates nuclear export, protects against exonuclease degradation, recruits ribosomes via translation initiation factors, and modulates translation efficiency. Poly(A) tailing improves stability by recruiting poly(A)-binding proteins, slowing degradation and extending half-life—and plays a crucial role in translation efficiency through interaction with the translation machinery.

The mRNA "body" is composed of four basic nucleotides: G, A, C, and U. Modification of the "body" through incorporation of chemically modified nucleotides significantly enhances translation efficiency, extends half-life, and reduces immunogenicity. The most prominent modification is pseudouridylation—where uridine (U) is converted to pseudouridine (Ψ) catalyzed by pseudouridine synthase.

In 2005, Katalin Karikó and colleagues reported that incorporating pseudouridine into RNA reduced immunogenicity in a dose-dependent manner. In 2008, they showed that complete replacement of uridine with pseudouridine dramatically reduced immunogenicity, increased stability, and enhanced translation. The two approved mRNA COVID-19 vaccines—mRNA-1273 (Moderna) and BNT162b2 (Pfizer–BioNTech) —both use N1-methylpseudouridine triphosphate (m1ψTP) instead of uridine triphosphate (UTP). As suggested above, CureVac's failure may have been due to its use of unmodified nucleotides.