Like a lot of misconceptions, this one germinated from a seed of truth -- from a statistical standpoint, you're most likely to come down with the flu in February, several months after the annual flu season actually begins in October or November [source: Brownstein]. And health authorities used to be concerned about the possibility that immunizations would wear off while the flu was still on the march.
But as Dr. James Conway, an associate professor of pediatrics at the University of Wisconsin School of Medicine and Public Health, explained to ABC News in 2008, we now know that isn't so. Instead, the immunity that you gain from the vaccine generally will last from October until the flu season ends in the spring. In some cases, a person's immunity may last for as long as a year [source: Conway].
So, physicians urge everyone to get their flu shots as soon as possible, so that we're protected throughout the entire flu season. But if you don't get a shot early, that's no reason not to go and get one later. It used to be that flu vaccines were only available during a tight time window in October and November, because health authorities felt they had to focus on high-risk groups, such as the elderly, before limited supplies of vaccines ran out. But in recent years, they've greatly expanded immunization efforts, and pharmaceutical companies have ramped up their production of vaccines. Some doctors begin receiving vaccine shipments as early as August [source: Brownstein].