evidence: Weissman was especially intrigued by a single-stranded molecule called messenger RNA, or mRNA, which brings our cells the DNA blueprint for making proteins so that the body can function. If we could manipulate those instructions, could mRNA be harnessed to create an entirely new kind of vaccine—one that could generate immunity without ever bringing a pathogen into the body? [...] From the start, Weissman and Karikó believed mRNA was the key to unlocking a new generation of vaccines and therapeutics. Theoretically, it could instruct any cell in the body to make any desired set of proteins. But practically, there were many obstacles. Synthetic mRNA was notoriously unstable and tended to break down before it could do its job. The closest attempt came in 1990 when researchers from the University of Wisconsin showed that injected mRNA could generate proteins in mice. Many scientists, however, were skeptical that this process could be replicated in humans. [...] The Pfizer-BioNTech and Moderna vaccines deployed the same clever mechanism. A shot of specially coded mRNA would instruct certain cells to manufacture the notorious COVID-19 spike protein, enabling the cells to briefly masquerade as the virus and teach the immune system to recognize it. Within weeks of injection, the mRNA would break down naturally without a trace, leaving in its wake a powerful immunity against the coronavirus.