Giving vaccines without needles (needle-free vaccine delivery) may be better than giving them using a needle for many reasons. One method for giving shots without needles is a technique called jet injection. This is what the researchers are testing in this study. A jet injector pushes the vaccine fluid out of a small hole at a high enough speed to allow the vaccine to go under the subject's skin without needing a needle. Years ago, people got shots using jet injectors, but these older devices reused the same "nozzle" or hole through which the fluid was forced. Newer jet injectors, including the one the researchers are testing in this study, use disposable cartridges to hold the vaccine. So, the only thing that touches the subject's skin is their own cartridge, which gets thrown away after injection.