Animal influenza viruses, such as the H1N1 and H5N1 viruses, typically infect animals like birds or pigs, but these viruses can also infect humans. The recent outbreak of the H1N1 influenza virus demonstrates that it is possible for these viruses to cause a pandemic outbreak in humans. Researchers are specifically interested in evaluating vaccines for H2 viruses, as these viruses caused a pandemic outbreak in humans in the late 1950s to 1960s. In addition, over half of the world's current population has no previous exposure to related viruses, which means a pandemic outbreak could pose a serious health risk as many people would have no immunity against the virus. In 2006, an outbreak of H2N3 infection occurred in pigs in the Midwest United States. While no humans contracted the virus during that outbreak, it remains a concern for researchers in the event that the virus ever spreads to humans. The purpose of this study is to evaluate the safety and immunogenicity of two doses of an experimental H2N3 vaccine, H2N3 MO 2006/AA ca in healthy adults. This is a live attenuated vaccine, which means that participants will be exposed to small amounts of the H2N3 vaccine virus and will need to remain isolated for at least 12 days when receiving the vaccine to prevent spreading the virus to others.