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It took ages to find the report below after the CDC removed it from their rabies site -- you might find it interesting.
The following quote is from the Center for Disease Control's MMWR Recommendations and Reports March 22, 1991/40(RR03);1-19
Rabies Prevention-- United States, 1991 Recommendations of the Immunization Practices Advisory Committee http://www.cdc.gov/mmwr/preview/mmwrhtml/00041987.htm "A fully vaccinated dog or cat is unlikely to become infected with rabies, although rare cases have been reported (48). In a nationwide study of rabies among dogs and cats in 1988, only one dog and two cats that were vaccinated contracted rabies (49). All three of these animals had received only single doses of vaccine; no documented vaccine failures occurred among dogs or cats that had received two vaccinations. " .