A further study compared anthrax vaccine delivery using ID microneedle immunization, IM injection, intranasal delivery, and epidermal delivery by microabrasion into mice (10 lg dose) and rabbits (50 lg dose). Microneedle ID vaccination showed slightly better response in the murine model than the other routes used, while all treatments in the rabbit had similar responses. A follow-up ID immunization was performed to compare ID injection with IM injection over a range of doses in a rabbit model: 10, 0.2, and 0.08 lg of anthrax vaccine (Mikszta et al. 2006) . After prime immunization, ID injection showed significantly higher immunogenicity than IM injection when using 10 and 0.2 lg dosages. Interestingly, ID injection with 0.2 lg showed a statistically equivalent response to IM administration of the 10 lg dose. After administration of a booster immunization, this dose-sparing phenomenon continued. Furthermore, an aerosol lethal challenge with anthrax spores showed that a 10 lg ID injection completely protected the immunized rabbits, whereas IM injection of the same dose protected only 71% of the rabbits.