Thiomersal is an organic mercurial compound that has been used for over 60 years as an antimicrobial agent to prevent bacterial and fungal contamination. Due to public concern about the possible toxicity of thiomersal, several regulatory agencies, including the European Agency for Evaluation of Medicinal Products and the Food Drug Administration, issued statements calling for removal of thiomersal from vaccines. Two randomized studies were performed to evaluate the effects, if any, of the complete removal of thiomersal on the safety and immunogenicity of FLUAD(R) and AGRIPPAL(R) influenza vaccines.