Redox status of the vaccinated and control groups throughout the course of the challenge period. The GSH/2GSSG ratio was determined for each dog using purified frozen red blood cells. After their lysis, the supernatant was collected and both GSH and GSSG levels were measured by the glutathione reductase enzyme recycling and modified method. The data are presented as the mean ratio between the reduced (GSH) and oxidised (GSSG) forms in each group with the standard error of the mean. The groups were significantly different at each time point from week 73.