Low-Temperature Gamma Irradiation Dosimetry for the Development of Streptococcus agalactiae Group B Vaccine Candidate Against Bovine Subclinical Mastitis

Gamma irradiation is a promising method for microbial inactivation in vaccine production, especially for heat-sensitive microorganisms, as it can preserve antigenic structures while ensuring pathogen safety. Because heat-sensitive microorganisms are prone to damage at room or elevated temperatures, low-temperature irradiation is essential to maintain protein integrity. A major challenge in this process is accurate dose measurement, as routine dosimeter calibration is typically performed at room-temperature, which can lead to inaccuracies under freezing conditions. In this study, polymethyl methacrylate (PMMA) dosimeters were calibrated under low-temperature environments maintained by dry ice (solid state of carbon dioxide; -700C to -300C). The influence of medium density on the absorbed dose was examined by comparing dry ice with wood pellets as a density-equivalent dummy. Dose-response data obtained at room-temperature using wood pellets were interpolated with the low-temperature data (using dry ice) to correct for temperature-dependent variations in dosimeter sensitivity. Results showed a polynomial relationship between dose and absorbance (R2=0.9996), with sensitivity decreasing above 10 kGy and saturation occurring near 50 kGy. At low-temperatures, dosimeter responses deviated from room-temperature calibration due to effects of dry ice and altered density. Correction was achieved by interpolating absorbance responses between dry ice (low-temperature) and wood pellets (room-temperature density-equivalent medium), and irradiator dose setting must be approximately 17.70% higher than the target dose to compensate for attenuation and achieve the desired absorbed dose. Dose uniformity ratios (DUR) across target doses (2–25 kGy) remained <1.1, confirming the homogeneity of irradiation and met the requirements of ISO 11137. This method provides a reliable dosimetric framework for low-temperature gamma irradiation, supporting its application in the development of vaccines against heat-sensitive microorganisms such as Group B Streptococcus agalactiae.