The CDC chooses at a particular time due to how long it takes to manufacture and release (FDA approval) the vaccine before the start of the flu season.
WHO wrote:
It takes approximately five to six months for the first supplies of approved vaccine to become available once a new strain of influenza virus with pandemic potential is identified and isolated. These months are needed because the process of producing a new vaccine involves many sequential steps, and each of these steps requires a certain amount of time to complete. The vaccine development process from start (obtaining a virus sample) to end (availability of vaccine for use) is summarized below.
Pandemic influenza vaccine manufacturing process and timelineTimeline is the same for all strains of influenza.
