Originally Posted By: Roarmeister
...I assume a different strain or else it takes more than a week to build up enough resistance in your system to be effective.

The general rule of thumb seems to be two weeks before you are protected, so it's possible that you were unlucky in your timing.

Or it might have been something else. Influenza is not the only bug to produce the typical "flu" symptoms. Or just another strain, as you mentioned. Or maybe you actually came down with one of the strains in the flu shot because no flu shot is 100% protective even with a match.