Rather than grow an influenza virus, we make only the small piece of the virus that is needed for immunity. This piece is called the hemagglutinin, or HA protein. We do this by taking the genetic information that tells cells how to make the HA protein and putting it into cultured cells that are grown in large stainless steel tanks. These cells act as powerful protein-producing machines and make hundreds of copies of the HA protein. We then purify these proteins and package them into Flublok. Each Flublok dose contains HA proteins that match the three most prominent flu viruses circulating during a given year, so that you are maximally protected.