Second Order Thermal Corrections to Vacuum Polarization in QED -- A   Covariant approach
  We study the electromagnetic properties of a hot medium at temperatures below electron mass. It was observed earlier that the first order hot loop corrections do not affect the electromagnetic properties of hot media due to the absence of self-interaction of photons. However, the second order contributions are found to modify these properties due to the overlapping hot and cold loops. It has also been noticed that the hot loops have to be integrated before the cold ones to establish the order by order cancelation of singularities to ensure renormalizability of the theory. It is also explicitly shown that the perturbative series of QED is still a convergent series in a covariant formalism at low temperatures. 
Samina S. Masood and Mahnaz Q.haseeb