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.
A microtiter plate (pictured) is used to perform the HI test. The plate contains wells (i.e., cup-like depressions that can hold a small amount of liquid), where a solution of antibodies, influenza viruses, and red blood cells are mixed and allowed to interact. These wells are in rows and columns which are identified on the microtiter plate by letters and numbers, respectively. The rows of the plate can be used to test different influenza viruses against the same set of antibodies.