Plenty of people skip the flu shot each fall. Some fear side effects. Others are too lazy or busy to get around to it.

But seeking out immunization is worth it, suggests a new study, which found that the influenza vaccine prevented millions of illnesses over a six-year period and kept more than 100,000 people out of the hospital.

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For the study, researchers from the Centers for Disease Control and Prevention collected national surveillance data from 2006 through 2011 on numbers of illnesses, rates of hospitalization, and vaccine effectiveness, among other factors.

Those numbers allowed them to estimate how many more illnesses would have occurred if nobody had been vaccinated.

Results of the number crunching, published today in the journal PLOS ONE, came up with an estimated 13.6 million illnesses prevented and nearly 113,000 hospitalizations averted over the study period.

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Every year is different with the flu and numbers of averted illnesses also varied year by year. In the winter of 2006-7, for example, the flu season was fairly mild and the vaccine prevented only 1.1 million cases. In the 2010-1 season, on the other hand, the vaccine kept 5 million people from getting sick.

“Vaccination against influenza has a substantial annual impact on the burden of disease in the United States,” the researchers wrote. Added lead researcher Joseph Breese in a press release: “These results confirm the value of influenza vaccination, but highlight the need for more people to get vaccinated and the imperative for vaccines with greater efficacy, especially in the elderly.”

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