It seems a little Rube Goldberg to induce the body to create spike proteins with mRNA, then have the immune system react to those newly-created spike proteins. Why not grow the spike proteins in a big vat (or in an animal or in culture) then inject the spike proteins into the body directly?
I can think of a number of reasons why mRNA might be better. But I’m not a biologist so I don’t know the hurdles in manufacturing and delivering spike proteins.
Disclaimer: Get vaccinated! I got my 2nd shot yesterday!
In a way, they are harder to produce because you have to make the proteins, collect them then inject them whereas the new vaccines let your body be the factory and delivery system. See, it’s not anymore Rube Goldberg because you still make the protein by inducing something to make it. Protein vaccine manufacture would induce perhaps yeast cells to produce spike and then you collect that for injection.
What you’re talking about is very much one of the tools in vaccine production.
Problems include allergic reaction to the host (egg, horse, bacteria or yeast), mutation (same as COVID mutation in humans), and simply, getting replication of a larger fragment is more difficult.
However, (not knowing any better), I think that this time, we got mRNA vaccines because there is a recently developed method of generating prototype mRNA vaccines, which is much faster and cheaper and generated a lot of different prototypes, with trials starting in ~ March last year (???) instead of March next year.
This mRNA technology was an awesome answer in search of its problem. It has a bunch of research lines with a some failures notably on cancer treatment. It’s my understanding that vaccines are a better fit for the tech because of the lower dosages needed.
As far as I know, mRNA technology was always built with an eye towards its use in vaccines, in part because of how much more quickly said vaccines could be made, and how they avoided problems that can arise in other methods.
Here is the SciShow video on this:
As for the question in the OP: having the host make the proteins is just a lot more efficient than making them and having to keep them around, and getting them into where they need to be.
You can do so by putting the RNA code in a virus instead, but that’s more complicated than just being able to use mRNA directly in something the cells will consume. There’s no danger of the immune system learning to attack the vaccine vector.
One mRNA can be run through the protein manufacturing system (the ribosomes) multiple times, so it’s not like you’d just replace one mRNA strand with one spike protein. Not that I think that was a factor in the OPs thinking.
There were something like 30 or more covid vaccines under development when i first started following it. They included mRNA (Moderna, Pfizer), viral vector (AZ, J&J, Sputnik), killed whole virus (Sinovac) and some that were just the spike protein with an adjuvant.
The ones we are using are the ones that made it to market and through testing first.
update: Novavax, a traditional vaccine that is basically the spike protein plus an adjuvant, did really well in clinical trials, and has been licensed by the Serum Institute of India, which hopes to produce 2B doses. It’s stable at cool temps, and doesn’t need to be frozen. I believe they’ve applied for FDA approval, although the FDA may not feel like it needs more emergency approvals, and might make them wait for full ordinary approval before it can be sold in the US
mRNA vaccines were under development for Ebola, and making some progress, but the Ebola epidemic blew over and the vaccine development got shelved. When Zika hit the scene, the technology development was dusted off and continued, I think. Then I guess that blew over too and the development was shelved.
So when Covid-19 hit, there was already some progress in making mRNA vaccines, so they just picked that up where it left off.