But, a new study out today is questioning their strength.
Watch the video above to hear 9NEWS medical expert Dr. John Torres explain.
You can get a flu shot, along with other free and low cost health screenings at this weekend's Fall Family 9Health Fair.
It's this Saturday from 7 a.m. until noon at the Barnum Recreation Center on Hooker Street in Denver.
Click here for information on the fair or others around the metro area this fall.
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