In April 2009, the United States began a response to the emergence of a pandemic influenza virus strain: A(H1N1)pdm09. Vaccination began in October 2009. By using US surveillance data (April 12, 2009-April 10, 2010) and vaccine coverage estimates (October 3, 2009-April 18, 2010), we estimated that the A(H1N1)pdm09 virus vaccination program prevented 700,000-1,500,000 clinical cases, 4,000-10,000 hospitalizations, and 200-500 deaths. We found that the national health effects were greatly influenced by the timing of vaccine administration and the effectiveness of the vaccine. We estimated that recommendations for priority vaccination of targeted priority groups were not inferior to other vaccination prioritization strategies. These results emphasize the need for relevant surveillance data to facilitate a rapid evaluation of vaccine recommendations and effects.