http://www.healthcare.gov/news/factsheets/2010/07/preventive-services-list.html#CoveredPreventiveServicesforAdults
this website says that flu shots are free under a network provider for insured individuals -
Immunization Services for Adults
adults covered are
"Adults 19 years and older who are enrolled in new group or individual private health plans will be eligible to receive vaccines recommended by the ACIP prior to September 2009 without any cost-sharing requirements when provided by an in-network provider as of September 23, 2010.
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