Hepatitis B vaccine and leukaemia
An abstract presented at the April 2002 meeting of the American Association for Cancer Research reported an association between hepatitis B vaccine and acute lymphocytic leukaemia in 334 children in northern California. The investigators suggested that thiomersal may play a role since its effects were more likely to occur with repeated dosing. The authors pointed out that the results should be interpreted with caution. The research suggested a link only between thiomersal in the hepatitis B vaccine and leukaemia. Other childhood vaccines containing thiomersal were not implicated.
Ethyl mercury, as thiomersal, is excreted rapidly from the body, mostly disappearing within 5–6 days. Such brief exposure to minute amounts of mercury is unlikely to be a sufficient carcinogenic stimulus to trigger childhood leukaemia. There is no other documented association between mercury and any human cancer, leukaemia, lymphoma, or other malignant or pre-malignant disease. In animal studies, cancer has been associated only with metal carcinogens involving continual or repeated exposure.
GACVS concluded that the suggestion of an association between hepatitis B vaccine and acute lymphocytic leukaemia, from one source and based on small numbers, was not convincing. The purported association might equally be explained by the design of the study concerned, which did not preclude the chance of statistical bias; the result does not provide a convincing causal link. Furthermore, since an association cannot be disregarded on the basis of available evidence, the Committee resolved that the matter should be kept under review. At this stage, the risk, if any, must be regarded as tentative at best and weighed against the proven benefits of hepatitis B immunization.
It is nonetheless important that the association with hepatitis B vaccine be further investigated using appropriate and robust epidemiological methods. In that regard, additional research is being conducted by the US Centers for Disease Control and Prevention (CDC) using the Vaccine Safety Datalink.
GACVS will continue to review the evidence and any epidemiological data that may emerge from ongoing studies and evaluate the possibility of analysing other registries.