close-up photography of person holding a blood samples on clear plastic tray

In this 2019 photo, a Centers for Disease Control and Prevention (CDC) scientist examines the results of a hemagglutinin inhibition (HI) test. HI tests can tell us whether antibodies, developed through vaccination, will also recognize circulating flu viruses. Using these data, scientists can decide which viruses to include in the seasonal flu vaccine. Photographer James Gathany