Everything not mRNA failed. Sometimes miserably with bad effects, and those effects have all been swept under the rug. Some of the vaccines gave people lifelong sensitivities to the adenovirus vectors. I can go on and on.

We got damned lucky that mRNA vaccines against Covid work as well as they do. Nobody new a priori (go look at Derek Lowe's writing from "In the Pipeline" during it all) and "everything would strike out" was not off the table.

And the mRNA stuff only worked because people already had been working on mRNA vaccines for other things slowly over decades. We got "lucky" that Covid appeared when we had all the pieces in places (liposome encapsulation, alternate amino acid replacement, etc.)

Before advocating for "fast track", advocate for better and more stable funding on the "slow track" pure research that takes decades but feeds into this kind of thing. The work of Katalin Karikó was instrumental in this stuff and yet she had to swim through mountains of shit to do the research and was denied tenure. With better funding, this stuff could have been done a decade earlier.