Still, going from "we know how this would work" to "we're actually using this in a wide-scale vaccination effort" seems like a pretty big step forward. These are the first mRNA vaccines licensed for use in humans.
You could say a similar thing about the atomic bomb: the science was well established, but actually building one was still a pretty big undertaking.
Fair enough. Special relativity was published in 1905, which suggested that enormous amounts of energy were potentially available from small amounts of matter, so the possibility of making a bomb was probably on the back of people's minds for a long time before someone actually built one. But yeah, as you say, they didn't really figure out the mechanics of how to build a bomb until quite a bit later.
1938 to the Trinity test in 1945 was about seven years.
Going by the wikipedia article on RNA vaccines, people have been experimenting with them since 1989, but there was a major advance in 2010 on getting mRNA into cells without triggering an immune response. Even then, there were a lot of attempts at treating various diseases and viruses that didn't pan out. So, it sounds like mRNA vaccines were kind of a long slow slog of running into problems and fixing them, whereas in the case of the atomic bomb, the idea was around for a long time but no one really knew how to get started until suddenly they did, and they immediately got busy building it.
You could say a similar thing about the atomic bomb: the science was well established, but actually building one was still a pretty big undertaking.