There is a theoretical risk of contamination of vaccines with yeast antigens with resultant mimicry between peptides of yeast and human myelin proteins. T-cells might be activated, with a resultant cross-reaction with myelin proteins. It has been suggested in the past that this might induce reactivity in patients with multiple sclerosis. However, in hepatitis B vaccine, for example, less than 1% of vaccine protein content is of yeast origin and there is no detectable immunogenicity. GACVS believes that the issue of mimicry should be considered with caution as there are few true examples in human disease of mimicry. Large amounts of yeast and adjuvants would be necessary for a reaction, and in general histocompatibility leukocyte antigen (HLA) binding studies are of uncertain significance. Humans are universally exposed to yeast in the environment and everyone will have antibodies against yeasts. Without a signal, there is little point at present in pursuing this theoretical concern. The data suggestive of a yeast-induced immune stimulation as a result of vaccine contamination are not convincing.

Full report of GACVS meeting of 2-3 December 2004, published in the WHO Weekly Epidemiological Record on 7 January 2005