Yeah, I think the shot is only 12 hours, or perhaps nothing at all.
Brett
I'll be willing to bet that the 'downing' criteria for immunizations is dependent on what is convenient at the time, and what is needed to ensure 'readiness' doesn't fall below the threshold that requires extra paperwork. (I'm still going to believe this even when feddoc proves me wrong.)
"Long weekend? Good, because the flu shot downs you for 72 hours."
"Flying tomorrow? Good, because the flu shot only downs you for 24 hours."
"Flying tonight? Good, because the flu shot only downs you for 12 hours."
"Walking to the bird? Good, because the flu shot isn't a downer."