Because influenza viruses are constantly evolving, and the viruses that circulate among people often change from one year to another, flu vaccines change annually. Scientists around the world collect samples to identify which flu strains are most likely to be circulating in the next flu season and here in the United States, we get our flu strains from the southern hemisphere.

In February—well before a new flu season begins—the U.S. Food and Drug Administration recommends the different strains of influenza viruses that should be included in vaccines that are going to be produced for the upcoming U.S. flu season starting in the fall.  These seed viruses are produced and then distributed to manufacturers to be grown and included in the upcoming flu vaccine. During this vaccine development, manufacturers and the FDA test for potency and safety, as well as quality control tests such as sterility.

The process of flu vaccine production stays the same every year and the only thing that changes is the seed strain of influenza that’s estimated to be circulating in the coming year.  The process used to produce pandemic vaccine (such as the H1N1 vaccine) is identical to the process for producing any other seasonal flu vaccine.

Once a seasonal or pandemic vaccine has been developed and tested, the lots are released in the summer for distribution and use by the public in the fall, typically when flu season begins.