Thiomersal allergy and vaccination reactions.

Thiomersal is the preservative in all toxoid vaccines routinely administered to children in the UK, but exposure from other sources is uncommon. Delayed hypersensitivity to thiomersal was demonstrated in 1% of individuals attending the Contact Dermatitis Investigation Unit, and 50 of these patients with positive patch tests to thiomersal were studied. Cross-reaction with other mercurials occurred in 17 of 29 patients tested (59%). 31 of the patients replied to a questionnaire regarding vaccination reactions, and were compared with case-controls matched for age, sex, and site of dermatitis. 4 patients in each group reported reactions to vaccines which contained thiomersal, suggesting that thiomersal hypersensitivity was not associated with an increased risk of vaccination reactions. However, individual cases of severe reactions to thiomersal demonstrate a need for vaccines with an alternative preservative.