  The vector channel spectral function at zero spatial momentum is calculated
at next-to-leading order in thermal QCD for any quark mass. It corresponds to
the imaginary part of the massive quark contribution to the photon polarization
tensor. The spectrum shows a well defined transport peak in contrast to both
the heavy quark limit studied previously, where the low frequency domain is
exponentially suppressed at this order and the naive massless case where it
vanishes at leading order and diverges at next-to-leading order. From our
general expressions, the massless limit can be taken and we show that no
divergences occur if done carefully. Finally, we compare the massless limit to
results from lattice simulations.
