Safety of thiomersal-containing vaccines

GACVS is maintaining a watching brief on the safety of thiomersal-containing vaccines. There is insufficient evidence to reach definite conclusions regarding the safety of thiomersal in possible special risk groups, notably malnourished infants and premature or low-birth-weight newborn infants. It is important to determine whether such individuals are at special risk, and WHO should encourage further research on the matter relevant to the developing world. Based on the most recent evidence, GACVS reported to WHO that there is no scientific basis for changing current WHO recommendations for thiomersal-containing vaccines, including administration of a birth dose of hepatitis B vaccine and vaccination of low birth-weight infants where indicated.

Full report of GACVS meeting of 11-12 June 2003, published in the WHO Weekly Epidemiological Record on 8 August 2003

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 USA government agency for methyl mercury. However, thiomersal contains ethyl mercury, not methyl mercury.

Expert advice and data presented to GACVS indicate that the pharmacokinetics of ethyl and methyl mercury are quite different. In particular, the half-life of ethyl mercury is short (less than 1 week) compared with that of methyl mercury (1.5 months). Thus, exposure to ethyl mercury in blood is relatively brief. Ethyl mercury is actively excreted via the gut, whereas methyl mercury accumulates in the body. Two independent epidemiological studies have recently been completed in the United Kingdom. One was funded by WHO (analysis of the General Practice Research Database (GPRD)), the other by the United Kingdom Department of Health (analysis of the data set of the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC). The GPRD analysis suggests that there is no association between developmental delay, particularly adverse neurological developmental outcomes or behavioural problems, and thiomersal-containing diptheria–pertussis–tetanus (DPT) vaccines given at 2, 3, and 4 months of age. These findings are supported by the ALSPAC results. These studies give further support to the safety in infants of thiomersal-containing vaccines in the amounts used in existing vaccines.

On this basis, GACVS concluded that there is currently no evidence of mercury toxicity in infants, children, or adults exposed to thiomersal in vaccines. It also concluded that there is no reason to change current immunization practices with thiomersal-containing vaccines on the grounds of safety.

Full report of GACVS meeting of 20-21 June 2002, published in the WHO Weekly Epidemiological Record on 22 November 2002