Flu season comes around like clockwork every year, and sooner or later everyone gets infected. The annual flu shot is a key part of public health efforts to control the flu, but the vaccine’s effectiveness is notoriously poor, falling somewhere from 40% to 60% in a typical year.
A growing body of evidence suggests that a history of exposure to influenza virus might be undermining the effectiveness of the annual flu vaccine. Partial immunity developed during prior flu seasons — either through natural infection or vaccination — might interfere with the body’s response to a new vaccine, such that vaccination mainly boosts the recognition of prior influenza strains but does little to create the ability to fight new strains.