Failure of protection induced by a Brazilian vaccine against Brazilian wild rabies viruses

Summary

This report shows that the SMB vaccine currently used in Brazil for human immunisation provides different degrees of protection in mice, depending on the rabies virus strain used as challenge. Using the NIH and Habel potency tests to evaluate the protective activity of rabies vaccine, we observed that vaccinated mice showed a higher resistance to a challenge with a fixed rabies virus (CVS – Challenge Virus Strain). The vaccine potency using the Habel or NIH tests was respectively > 6.4 (log 10) and 1.0 (Relative Potency-RP) when the fixed rabies virus was used for challenge, and from 2.9 to 4.3 (log 10) or 0.13 to 0.8 (RP) when different wild rabies viruses were used for challenge. The presence of virus neutralising antibodies (VNA) could not explain the differences of susceptibility after vaccination, since sera of vaccinated animals had similar VNA levels against both fixed and wild strains before virus challenge (respectively, 5.6 ± 0.24 and 5.0 ± 0.25 IU/ml of VNA against the fixed rabies virus and the 566-M strain of wild rabies virus in sera of mice vaccinated with 0.2 units of vaccine). Only cell-mediated immunity parameters correlated with the protection induced by vaccination. The IFN_γ titers found in sera and brain tissues of animals challenged with CVS strain were higher (from 36.7 ±  5.7 to 293.3  ±  46.2 IU/ml) than those found in mice challenged with 566-M virus strain (from 16.7 ± 5.8 to 36.7 ± 5.8). The proliferation index of spleen cells obtained with CVS stimulation reached a maximal value of 15.1 ± 0.7 while spleen cells from vaccinated mice stimulated with 566-M virus failed to proliferate. The implications of these data in human protection by vaccination are discussed.