A sealed pharmaceutical container includes a shoulder, a neck extending from the shoulder, and a flange extending from the neck. The flange includes an inclined sealing surface defining an opening in the sealed pharmaceutical container. The sealed pharmaceutical container also includes a sealing assembly including a stopper extending over the sealing surface of the flange and a cap securing the stopper to the flange. The stopper has a glass transition temperature (Tg) that is greater than or equal to −70° C. and less than or equal to −45° C. The sealing assembly maintains a helium leakage rate of the sealed pharmaceutical container of less than or equal to 1.4×10−6 cm3/s as the sealed pharmaceutical container is cooled to a temperature of less than or equal to −45° C.