Thimerosal-containing influenza vaccines were banned from use in California. The governor, Arnold Schwarzenegger, was reacting more to public advocacy groups than the weight of medical opinion. Thimerosal, containing an ethylmercury compound has been used in vaccines in microgram quantities to prevent life-threatening bacterial contamination. In 2004, a report by the Immunization Safety Review Committee of the Institute of Medicine stated that the benefits of vaccination are proven and the hypothesis of susceptible populations is presently speculative. After considering available medical studies from several countries, the report rejected any connection between thimerosal-containing vaccines and autism. Neverthless, some lay persons convince themselves otherwise.