https://www.vice.com/en_us/article/g5by4y/why-were-finally-closing-in-on-a-game-changing-universal-flu-vaccine

Flu season rolls around each year like clockwork, bringing with it the reminders to get your flu shot (consider this your reminder.) Every other vaccine requires a finite dosage and confers long-term immunity—it’s only the flu shot that must be taken annually.

While that may not seem like a big problem, having to get a vaccine annually is at best a hassle, and at worst a barrier. In fact, the U.S. hasn’t surpassed 50 percent flu vaccine coverage at any point in the past decade, according to data from the Centers for Disease Control and Prevention. Reaching 50 percent coverage is a critical threshold for herd immunity, which in theory prevents sustained transmission of the virus from occurring.