CDC reports that the vaccine failed miserably against a flu bug that popped up halfway through the season. This season's shot turned out to be a mismatch against the bug that showed up late in the year. The least effective flu vaccine in the past decade was in 2014-2015; that year a mismatch was also blamed. According to ABC News, vaccines against most infectious diseases are not considered successful unless they are at least 90% effective. Due to the flu virus' ability to change so quickly, vaccine effectiveness usually averages around 40%.