Thiomersal is more effective against bacteria , has better shelf life , and improves vaccine stability , potency , and safety , but in the U.S. , the European Union , and a few other rich countries , it is no longer used as a preservative in childhood vaccines , as a precautionary measure due to its mercury content . Thiomersal is more effective against bacteria , has better shelf life , and improves vaccine stability , potency , and safety , but in the U.S. , the European Union , and a few other affluent countries , it is no longer used as a preservative in childhood vaccines , as a precautionary measure due to its mercury content .