The synthetic mRNA used in the most successful COVID vaccines thus far is so delicate it has to be stored at ludicrous temperatures for transport/etc. prior to injection. And to protect it in the vaccine itself, it needs to be encased in lipid nanoparticles (kind of like tiny encapsulating oil drops) so it isn't just immediately destroyed upon entering your body and survives long enough to slip inside your muscle cells. I doubt it could exist in an aerosolized form long enough for anyone to inhale it, and I'm not even sure inhaling it would work. It needs to be able to enter enough of your body's cells to be transcribed and create the replica spike proteins in sufficient enough numbers for your immune system to respond.
I guess you might be able create an aerosolized viral vector vaccine by using a modified version of an existing airborne virus as the carrier (instead of the modified adenoviruses used for the AstraZeneca and other viral vector vaccines), but even ignoring the ethical dilemma of medicating people with things against their will (even if it's for their own good), that sounds like the beginning of zombie apocalypse film and I'm sure there are so many ways that could go biologically sideways that are too awful to even think about.
Edited by TM2-Megatron, 12 May 2021 - 05:59 PM.