Immunity to rabies after administration of prophylactic human diploid-cell vaccine

The immune status to rabies of 14 volunteers was determined using the commercially available Trousse Platelia Rage (Diagnostics Pasteur) enzyme immunoassay test system. Twelve subjects were evaluated before and between 6 months and 60 months after prophylactic intramuscular (deltoid) administration of rabies human diploid-cell vaccine, while the effect of booster doses on a further 2 volunteers was evaluated over an 11-year period. Optical density values were converted to international units to allow correlation with World Health Organisation seroneutralisation references. Values of greater than or equal to 0.5 IU are considered protective. The results showed that most individuals were still immune 2 years after vaccination; there was a tendency for serum antibody levels to decrease over a 5-year post-vaccination period. Antibody levels rose sharply after booster immunisation, after which they decreased at a much slower rate. In general, results revealed that after the first booster, additional booster vaccinations at 5-yearly intervals would provide adequate prophylactic immunity. There was, however, much individual variation, which emphasises the need to evaluate each individual at regular and shorter intervals to determine the need for booster vaccine doses. The test method employed is economical and well suited to such evaluations.