  We study the influence of running vacuum on the baryon-to-photon ratio in
running vacuum models (RVMs). When there exists non-minimal coupling between
photons and other matter in the expanding universe, the energy-momentum tensor
of photons is no longer conserved, but the energy of photons could remain
conserved. We discuss the conditions for the energy conservation of photons in
RVMs. The photon number density and baryon number density, from the epoch of
photon decoupling to the present day, are obtained in the context of RVMs by
assuming that photons and baryons can be coupled to running vacuum,
respectively. Both cases cause the baryon-to-photon ratio to no longer be a
constant. However the evolution of the baryon-to-photon ratio is strictly
constrained by observations. It is found that if the dynamic term of running
vacuum is indeed coupled to photons or baryons, the coefficient of the dynamic
term must be extremely small, which is unnatural. Therefore, our study
basically rules out the possibility that running vacuum is coupled to photons
or baryons in RVMs.
