It's quite possible the biggest issue with the COVID vaccines we have is that intramuscular injection produces a blood-borne immune response but the method of infection is through the lungs, and you get much less response there.

It's why there's been a lot of interest in inhalable vaccines[1] although getting them to market has had a lot of delays.

[1] https://www.nbcnews.com/health/health-news/nasal-covid-vacci...