The purpose of this research study is to find out if giving the smaller dose of flu vaccine under the skin generates antibodies against flu compared to giving the vaccine the usual way, as a shot in the arm. If using smaller doses in this manner is effective, the current supply of vaccine could be used to make more doses to give to more people. About 217 healthy adults, 18 to 49 years of age, will participate. The study will be conducted at one site in the United States and subjects are expected to participate for about 6 months. Blood samples will be taken to assess the immune system response. Local and systemic safety will be evaluated in the 28 days following vaccination.