A lot (boldface mine):

These high hospitalization rates equated with a substantial influenza burden of illness during the 2016–2017 season with an estimated 30.9 million people getting sick with influenza, 14.5 million going to a health care provider for influenza, and an estimated 600,000 people being hospitalized from influenza…. Overall, the burden estimates for last season were approximately as high as the estimated burden for 2012–2013 and 2014–2015 seasons…

Improvements in vaccine coverage could provide a greater public health benefit. For example, if vaccination rates increased by just 5 percentage points across the entire population, another 483,000 illnesses, 232,000 medical visits, and 6,950 hospitalizations associated with influenza could be prevented. If vaccination rates improved to the Healthy People goal of 70% for all age groups, another 1.89 million illnesses, 822,000 medical visits, and 17,300 hospitalizations could have been prevented during the 2016-2017 influenza season…

CDC estimates that influenza vaccination during the 2016–2017 influenza season prevented an estimated 5.29 million illnesses, 2.64 million medical visits, and 84,700 hospitalizations associated with influenza. This report underscores the benefits of the current vaccination program, but also highlights areas where improvements in vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health. For example, increasing vaccination coverage among working-age adults younger than 65 years would further reduce the burden of influenza, as this age group continues to have the lowest influenza vaccination coverage.

When looked at individually, the flu vaccine doesn’t seem that helpful, but at a national level, it really does make a difference.