  In a recent paper (Eur. Phys. J. C 80: 68 (2020).), a definition of
oscillating neutrino states in quantum field theory was proposed. We show that
such definition can be derived in a particular case of the Blasone-Vitiello
approach, when mass vacuum is chosen as the physical vacuum. We discuss some
problems of such an approach, which appears to be mathematically inconsistent
and physically not acceptable.
