since the 1930s some vaccines have contained the preservative thiomersal. in the body, this is changed to ethylmercury. at first it was thought that this mercury-based preservative can cause or trigger autism in children, but scientific studies could not show such a link. because of this, thiomersal has been removed from most u.s. vaccines recommended for children six years of age and under. there are certain exceptions to this rule for influenza vaccines. in some cases, vaccines may still have very small amounts of thiomersal in them.