Immunity to the flu virus may wane over the course of the flu season, so you don’t want to get a flu shot too early, or too late.
Q. Does the flu vaccine become less effective over the course of the flu season? If yes, should I delay getting my flu shot until later in the season?
A. Yes, immunity to the flu virus may wane over the course of the flu season, so you don’t want to get your flu shot too early, such as in the summer months. But you also don’t want to get it too late, once flu season starts. For most people, sometime in October is best.
In 1990, the Immunization Practices Advisory Committee, the branch of the Centers for Disease Control and Prevention that issues vaccine recommendations, warned that, in individuals 65 years of age and older, “it is particularly important to avoid administering vaccine too far in advance of the influenza season because antibody levels begin declining within a few months.”
Since then, the question of waning immunity has been the subject of several studies, but results have been inconsistent.