It is true that the flu vaccine doesn't always work. According to the CDC, it is effective in preventing a recipient from getting the disease about 60 percent of the time. How well it works depends upon your age and your health; young, healthy people are most likely to get the desired results, according to Dr. Bill Schaeffner, chairman of the preventative medicine department at Vanderbilt University. Another factor is how well this year's vaccine matches to the flu strain that emerges as a threat [source:CNN].
That last part is a continuing challenge for health authorities, because the flu isn't caused by just one virus that remains the same every season. Instead, different strains of the flu continually evolve, and more than one strain may spread simultaneously. That's why each year the vaccine is concocted from three strains of influenza, the ones that researchers guess will probably be the most active, based upon data from previous years [source: Lowrey].
But even if you get a flu shot and still get the flu, your precaution didn't go entirely to waste. The vaccine can help to protect you against some of the disease's harsher complications, experts say [source:CNN].
One important caveat: There are some people who should not get a flu shot without first consulting with their physician. That list includes people who are severely allergic to chicken eggs, those who already have a moderate-to-severe illness with a fever (they should wait to get healthy before getting the vaccine) and those with a history of Guillain-Barre Syndrome, a paralytic illness [source: CDC].