All available flu vaccines in the United States this flu season are quadrivalent (four-component) flu vaccines that are designed to protect against the four flu viruses that research indicates are most likely to spread and cause illness among people during the upcoming flu season. In its role as a World Health Organization (WHO) Collaborating Center, CDC conducts year-round surveillance of circulating flu viruses and uses this and other data to assess population risk and to recommend the vaccine viruses to include in vaccine production for both the Southern and Northern Hemispheres. Flu viruses constantly change and, based on the available data on how flu viruses are currently changing, CDC flu experts anticipate that the flu viruses that circulate this season may have small changes in comparison to each of the four major virus components used in the vaccine. Despite these changes, the flu vaccine is still expected to provide good protection for the majority of flu viruses that are likely to circulate in the United States during the upcoming flu season. Since flu viruses change rapidly, some changes can result in decreased similarity between circulating viruses and one of the vaccine’s four components. The similarity of the vaccine’s four components to the flu viruses that circulated during previous seasons is just one of several types of data used to inform the recommendations for the composition of flu vaccines. Other data include the extent and location of flu virus circulation and the level of immunity in the population provided by the prior season’s vaccine against recent circulating flu viruses. CDC constantly reviews the composition of flu vaccines and suggests updates when needed to keep up with the latest flu viruses. Sometimes, updates to the composition of the flu vaccine represent minor, incremental improvements. Other times, the updates to the vaccine can represent more significant changes intended to provide optimal protection against flu viruses that have evolved to evade people’s existing immunity.