In many text books on relativity, one finds at some point a statement that the vacuum stress energy tensor should be Lorentz invariant, from which it then follows that the vacuum pressure is minus the vacuum energy density. However, the vacuum energy density (or stress tensor) is not an observable as such, so it is not clear to me why it should be Lorentz invariant. Only when it is included in some physical law, then this law should be Lorentz invariant (or so called covariant). An invariant law does not have to imply the invariance of all its components. Is there a flaw in my reasoning ?