Statement on thiomersal

The Global Advisory Committee on Vaccine Safety concludes that there is no evidence of toxicity in infants, children or adults exposed to thiomersal (containing ethyl mercury) in vaccines.

In 1999, concerns were raised in the United States of America about exposure to mercury in vaccines. This was based on the realization that the cumulative amount of mercury in the infant immunization schedule potentially exceeded the recommended threshold set by the United States government for methyl mercury. However, thiomersal, the preservative in some vaccines, contains ethyl mercury not methyl mercury. The Global Advisory Committee on Vaccine Safety (GACVS) first assessed this issue at a special meeting in August 2000. The Committee review has been ongoing since then.

Expert consultation and data presented to the GACVS indicate that the pharmacokinetic profile of ethyl mercury is substantially different from that of methyl mercury. The half-life of ethyl mercury is short (less than one week) compared to methyl mercury (1.5 months) making exposure to ethyl mercury in blood comparatively brief. Further, ethyl mercury is actively excreted via the gut unlike methyl mercury that accumulates in the body.

Four independently conducted epidemiological studies investigating associations and frequency of neurobehavioural disorders in relation to vaccination with thiomersal-containing vaccines have been completed in the United Kingdom of Great Britain and Northern Ireland and Denmark. The findings from these studies do not challenge the safety of existing thiomersal-containing vaccines in infants. Recently two studies were published alleging reduction of neurodevelopmental disorders in the United States of America following discontinuation of thiomersal-containing vaccines in the national immunization programme. The Committee found the conclusions made by the authors unconvincing due to the study design, and the data source.

The GACVS reviewed available information on an ongoing thiomersal pharmacokinetic study in macaque monkeys and assessed the validity of animal models in studying associations between thiomersal and neurobehavioural disorders in humans. The Committee was informed of ongoing human neurobehavioural studies and thiomersal exposure in the United States of America and Italy and of a study on the suitability of thiomersal-free vaccines in multidose vial presentations, assessed by retained sterility for up to 30 days.

On the basis of the foregoing, the GACVS concluded that the most recent pharmacokinetic and developmental studies do not support concerns over the safety of thiomersal (ethyl mercury) in vaccines. The Committee concluded, and advises accordingly, that there is no reason on grounds of safety to change current immunization practices with thiomersal-containing vaccines, as the risks are unproven. However, data for well-nourished neonates born at term cannot necessarily be extrapolated to preterm or malnourished infants. Studies on the latter group would be difficult to conduct, but the GACVS encourages further research.

The GACVS will continue to review the evidence, including any epidemiological data, that might emerge from on-going studies.

The GACVS is a scientific advisory body established by WHO to provide a reliable and independent scientific assessment of vaccine safety issues in order to respond promptly, efficiently and with scientific rigour to such issues. Membership includes experts from around the world in the fields of epidemiology, paediatrics, internal medicine, pharmacology and toxicology, infectious diseases, public health, immunology and autoimmunity, drug regulation, and safety.