“…Phenol red, a colorimetric dye used in tissue culture media to monitor pH during fermentation, had a significant stabilizing impact on the sIPV2 at lower pH. 47 Freezing, caused either intentionally by storage at −20°C or inadvertently by improper cold storage, resulted in a significant reduction in D-antigen content. 48 For example, the D-antigen content measured in vaccine samples stored at −20°C for 1-day, or 1-week, or after mimicking improper cold storage for 3 freeze–thaw cycles was significantly reduced.…”