I believe that the technocratic culture of US medicine is standing in the way of research on intranasal vaccines.
What you consistently hear from medical professionals is, "we have this proven technology, it works, the objections to it are irrational, can we please just be sensible?" So far, so good… (I'm a bit of a technocrat myself)
But why are people so afraid of vaccines in particular? There are all sorts of arguments about autism and turning frogs gay or whatever, but people are still happy to take whatever pills the doctor prescribes and hormone-enriched fast food… what makes vaccines different? It seems pretty clear that people are just afraid of needles, and from an evolutionary biology perspective, that's not irrational at all!
But "maybe we should make vaccines that won't scare people" just won't get you anywhere in the US medical establishment. Frightened patients aren't Serious Medicine.