How did i contract the flu when i got a flu shot already?

Various strains yrly: There are at least 4-8 strains of flu circulating the world yearly.International plane travel assures us the possibility of imported strains on almost daily basis.The vaccine has the 3 most common strains based on the predictions made when the vaccine starts into production.The vaccine will only cover the strains in the shot, so it's possible to get flu from an alternate strain.