Dug this up on Lallemand website:
Response from Dr. Clayton Cone:
Active Dry Yeast, at <5% moisture, is originally packaged in an oxygen free atmosphere either via nitrogen flush or vacuum. Under these conditions they will loose about 20% activity / year when stored at 20C.(68F) and about 5% activity / year when stored at 4C.(40F). It is the presence of Oxygen and the pick up of small amounts of moisture that causes the yeast to deteriorate at a faster rate, once the package has been opened. If you can vacuum pack (kitchen vacuum package equipment)or store in an air tight container and refrigerate you may retain a substantial amount of the activity. It would be wise to increase the inoculum 50% to be on the safe side. It is always a gamble, depending on how much moisture the cells have picked up each time you open and close the package and return to the refrigerator.
I'd still vote for getting new yeast though.