Preservatives in vaccines, such as thiomersal, phenoxyethanol, and formaldehyde, prevent serious adverse effects. Thiomersal is more effective against bacteria, lasts longer in storage, and makes the vaccine stronger, safer, and more stable (less likely to be changed by things like heat). However, in the United States, the European Union, and a few other developed countries, it is no longer used as a preservative in childhood vaccines because it contains mercury. Some people have argued that thimerosal contributes to autism. However, no convincing scientific evidence says this is true.