I saw a blurb about tests of a two part vaccine on mine. First dose an mRNA vaccine injection. Second dose is intranasal naked spike protein. Provided good protection when the mice were challenged. The spike protein isn't infectious. Part of the motivation behind intranasal vaccines is the hope that mucosal immunity provides better protection.

Also read a summary of influenza challenge studies. People exposed to low doses often seroconverted asymptomatically.

I feel it's plausible small exposures after vaccination would build more protection.