Anecdata alert!! I've been getting the flu shot every year for almost 10 years.  I've been exposed to a lot of people with the flu.  I haven't had the flu yet.  They make a best educated guess at which strains will be most active in a given flu season.  There are usually three strains the vaccine will protect against.  Sometimes, though, they don't guess correctly or there may be a different strain that's more prevelant in one area. Having said that, I don't get too up in arms about people not vaccinating for the flu.  Other diseases like polio, rubella, mumps, measles, etc.?  Yes, vaccinate or go to some isolated island somewhere.