The effect of residual moisture in lyophilized smallpox vaccine on its stability at different temperatures.

In a study of the influence of residual moisture on the potency of freeze-dried smallpox vaccine, vaccine lots with moisture contents between 0.36% and 6.7% were used. These lots were stored at 4 degrees C, 24 degrees C, and 37 degrees C for up to 6 months, or were exposed to 100 degrees C for up to 3 hours, and were sampled at regular time intervals for assay. No loss in potency occurred at 4 degrees C where the moisture content was 4.8% or less. At 24 degrees C the critical moisture level was 2.5%, and at 37 degrees C it was 0.36%. At 100 degrees C the potency loss was more rapid and there were significant losses after 1 hour at a moisture level of 2.4% or more. A residual moisture content of less than 1% seems to be essential for satisfactory storage of freeze-dried smallpox vaccine.