Booster effect of human diploid cell antirabies vaccine in previously treated persons

Contact was maintained with 27 of 45 persons for four years after their severe exposure to rabies virus and successful protection. During that time, these persons showed no adverse effects of treatment and maintained rabies-neutralizing antibody at the level of 2 to 91 IU/mL of serum. After a single booster inoculation of human diploid cell culture vaccine four years after the initial treatment, levels of neutralizing antibodies rose to 36 to 650 IU/mL of serum ten days after the booster inoculation. These observations confirm the persistence of rabies-specific serum antibodies for an extended (four years) period after vaccination. Furthermore, a single booster inoculation with human diploid cell culture vaccine resulted in a sharp increase in the level of antibodies, indicating that a person would probably be protected against a second exposure.