Thimerosal is a compound that has played an important role either as a preservative or in the initial stages of the manufacture of some vaccines for over 60 years. It is included in the pandemic vaccine, Pandemrix, in order to prevent bacterial and fungal contamination of the vaccine when the multidose vial is opened.
The mercury content of thimerosal has led to concerns that it may affect brain development when given in vaccines. Based on a large amount of scientific data, the WHO, the United States Institute of Medicine and the European Medicines Agency have concluded that the evidence favours no link between thimerosal-containing vaccines and autism.
Pandemrix also contains an adjuvant. An adjuvant is a substance that enhances the immune response to the viral protein. Benefits of using an adjuvant include better cross-protection from other influenza strains and also means that less viral material is used in each dose so that more vaccine can be produced. This adjuvant has been tested in clinical trials involving several thousand people and the safety profile is considered to be acceptable.