Ironically, an oral vaccine (as long as it is not some sort of mRNA abomination) could actually be somewhat effective. The issue with vaccines for respiratory viruses is that there are separate compartments to the human immune system. Primarily the mucosal and systemic compartments. Mucosal immunity informs the systemic immune system, but not the other way around. So giving an injected "vaccine" against a respiratory virus will only provide systemic immunity at best. It cannot prevent contraction or spread. An oral vaccine or a nasally sprayed vaccine, on the other hand, interacts directly with the mucosal immune system, triggering the development of IgA antibodies, rather than the systemic IgG antibodies. Though, for an RNA virus like a corona virus, this is likely pointless as they mutate rapidly, so the vaccine is often out of date by the time it is produced. The best method for gaining robust immunity to a corona virus is to contract it, they are common and generally mild unless there are significant underlying comorbidities. I wouldn't go for a specific vaccine (oral or not) for a respiratory virus anyway. Much better effects can be gained by using killed micobacterium vaccae (a soil bacteria). This has the effect of essentially resetting the innate immune system, restoring its vigor and improving the body's immune response to all manner of threats. This kind of treatment, aimed at getting the body's immune system to work properly is much more promising, though much less profitable, as it is very inexpensive to make, not patentable and prevents myriad illnesses at a stroke.