There exist successful veterinary coronavirus vaccines, and an optimist (at least, a sufficiently cynical one) would posit that we don't have any human examples

There exist successful veterinary coronavirus vaccines, and an optimist (at least, a sufficiently cynical one) would posit that we don't have any human examples because it has not previously been deemed sufficiently profitable (or, more charitably, an efficient use of resources better spent on more pressing issues) by whoever makes funding decisions for vaccine development, not because of a failure of science. Edit: That being said, efforts to develop a SARS vaccine were quite horrifying on whole, so there's certainly no natural law that says a vaccine must exist to be invented/discovered.