Suppliments: You need a balance.
I for one was a strong proponent of Vitamin C for helping prevent illness, but I have since heard that they have found no real positive link between increased ingestion of C and the occurance of diseases. It does seem to help get rid of the bug, but it does not make it so you do not get it in the first place.
That being said, if you have DEFICIENCIES in any of your nutrients, your system may be compromised and you may catch things easier or get them worse, but that is another matter.
Now, vaccines? They try to immunize for the strain that is coming through, and they usually get it right enough to prevent many cases. The thing I am not sure about is similar strains, whether some viral immunities may give a partial resistance to others because of their similarity. I do not know if this is entirely true (the bonding of antibodies to antigens that do not exactly meet the same composition as the one you are currently infected with...) Kind of like two pieces that do not fit exactly, but still kind of fit....
THAT I am not sure of.
But saying that you have had the flu and been fine is BS. you do not walk around all hunkey dorey with a 104 fever. What, you didn't HAVE a 104 fever? Guess what, you did not have the flu. (OK, some may only be 102...)
Also, as others have said, unless you are behind enemy lines under fire or somehow transported back in time to WWII behind the lines in France, you should NOT be in ANY operating capacity while exhibiting symptoms. You could HURT OTHER PEOPLE because of your diminished capacity and you could also spread the disease. It would NOT be smart for the armed forces to sed you out with anything more serious than the sniffles unless it was an immediate threat.
I myself have been very sick at times. Feeling all wonky and running like a faucet while still going to work (no real "sick" days here unless you are out 3 or more days in a row... and I think you need a Dr's note). But I know it wasn't the flu (no high fever).
That said, if you are in a profession with a LOT of contact with people, like teaching, MAN UP and take your damn shot ya wuss! You stand a higher chance of catching and spreading it, and with cases like Teaching or Emergency Care, we would all rather have you feeling ookey for a week than be completely out for the same time along with a half dozen other people.