Why is the spin projection operator (1−γ₅s̸)/2 covariant?

[LayMuon]

I cannot quite understand why expression $\frac{1-\gamma_5 \slashed{s}}{2}$ is covariant? We defined it in the rest frame, and then said that because it is in the slashed expression, it's covariant, what does that mean? s is the direction of polarization, $s \cdot s = -1$

 

[LayMuon]

Figured it out, one should use the identity $\hat{S}(\hat{a}) \gamma^\nu \hat{S}^{-1} (\hat{a}) = a_\mu ^{\text{ } \nu} \gamma^\mu$, the rest is straightforward.