Vaccine Access, Certainty, and Innovation Act of 2015 This bill requires the Centers for Disease Control and Prevention (CDC) to report on the process used by the Advisory Committee on Immunization Practices (ACIP) in making recommendations on vaccines. The Department of Health and Human Services (HHS) must issue final guidance to facilitate the use of expedited pathways for the development and licensure of vaccines to prevent certain infectious diseases. The Public Health Service Act is amended to require the CDC to provide a vaccine developer, upon request, with information on public health needs and priorities and certain epidemiological analyses or data. The Federal Food, Drug, and Cosmetic Act is amended to require the Food and Drug Administration to establish a process for designating diseases as tropical diseases included in the priority review voucher program. The amount of the priority review user fee is set to the cost difference between a priority review and a non-priority review. The National Institutes of Health must support or conduct activities to advance the development of vaccines and must report on vaccine research. Within 60 business days of a recommendation by ACIP, HHS must determine whether to update Medicare coverage of vaccines. The Government Accountability Office must report on the impact of Medicare and Medicaid reimbursement rates on vaccine access. For purposes of calculating a medical loss ratio, the cost of a program to increase adult immunization is accounted for: