Annual vaccination is the main method for preventing seasonal influenza, which typically occurs in the United States from late fall to early spring. Manufacturers produce vaccine through a lengthy and complex process. Manufacturers and medical supply distributors then ship vaccine to providers such as physicians. Each year, the Department of Health and Human Services's (HHS) Centers for Disease Control and Prevention (CDC) recommends who should be targeted for vaccination, including those at higher risk for influenza-related complications or medical care--for example, adults aged 50 years and older, young children, and some individuals with chronic medical conditions. CDC bases its recommendations on those made by the agency's Advisory Committee on Immunization Practices (ACIP). GAO examined: (1) factors that affect the quantity of vaccine produced and when it reaches providers, (2) issues related to making vaccine available to high-risk and other target groups, and (3) public health messages produced and disseminated by CDC and others to promote vaccination. GAO reviewed relevant documents and interviewed officials from CDC, other public health entities, manufacturers, and medical supply distributors, and examined data on vaccine doses produced and shipped.