Global Vaccine Safety

Position of the Global Advisory Committee on Vaccine Safety regarding concerns raised by paper about the safety of thiomersal-containing vaccines

May 2003

A publication in the Journal of American Physicians and Surgeons 1, claims evidence of a link between mercury exposure from thiomersal-containing childhood vaccines and neurodevelopmental disorders and heart disease.

Based on a careful preliminary review, the Global Advisory Committee on Vaccine Safety (GACVS) finds that the article does not provide a sufficient scientific basis for changing the WHO policy in respect of thiomersal-containing vaccines. However, WHO and GACVS will continue to keep the issue under careful and ongoing review.

The article has a number of limitations which undermine the conclusions drawn by the authors. These include: inaccessibility to the reader of the data on which the analysis was made; lack of clear case definitions for the conditions referred to in the paper; unclear or insufficient description of applied statistical methods; an assumption made by the authors that the toxicity of ethyl-mercury is equivalent to that of methyl-mercury, an assumption that cannot necessarily be made, and against which various authorities have warned; the assumption in the paper that the populations under study are the same (there is every possibility in the methods used of selection bias); and a failure to account for changing reporting patterns for diseases attributed to the vaccines over the years of the study. Published outcomes of the study regarding neurodevelopment and heart disease following administration of thiomersal-containing vaccines do not meet the scientific criteria required to suggest causal relationship. These points, taken together, lead GACVS to conclude that the paper provides insufficient evidence to warrant changes to public health policy.

Background

In 1999, concerns were raised in the United States of America regarding exposure to mercury following immunization with thiomersal-containing vaccines. This was based on the calculation that the cumulative amount of mercury in infant immunization schedules potentially exceeds the recommended threshold set by a United States government agency for methyl mercury. However, thiomersal contains ethyl mercury, not methyl mercury.

Expert advice and data presented to the GACVS in June 2002 indicate that the pharmacokinetics of ethyl and methyl mercury are quite different. In particular, the half-life of ethyl mercury is short (less than one week) compared with that of methyl mercury (1.5 months). Two independent epidemiological studies completed in the United Kingdom of Great Britain and Northern Ireland suggest that there is no association between developmental delay, particularly adverse neurological developmental outcomes or behavioural problems, and thiomersal-containing diphtheria–pertussis–tetanus (DPT) vaccines.

On this basis, the GACVS has concluded that on present evidence it cannot be concluded that thiomersal in vaccines is associated with mercury toxicity in infants, children, or adults. The literature, including the most recent published article referred to above 1, provides no reason for change in current immunization practices with thiomersal-containing vaccines on the grounds of safety 2.

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.

  • Geier MR, Geier DA. Thimerosal in Childhood Vaccines, Neurodevelopmental Disorders, and Heart Disease in the United States. Journal of American Physicians and Surgeons, 8(1):6-11, 2003.
  • Global Advisory Committee on Vaccine Safety, 20-21 June 2002. Weekly Epidemiological Record. 77(47):389-394, 2002.
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