Never had one so far. Not sick in years.
Some believe that immune systems get lazy when antibodies are fitted with crutches (like mega doses of vitamins or artificial antibody stimulators).
Healthy lifestyle ( with the exception of hanging out here
) and normal exposure the the urban mix of traditional and designer diseases (courtesy of medicine/hospitals) seems to do the trick for me.
The idea that vaccinations work by creating a 'controlled blast' of the disease of the month to kick start your immune system never sat well with me. If you look at more extreme vaccinations like the one for Malaria and its disclaimers it does not help ease the mind.
The flu shot does seem to have its place, but even for expecting moms and young children, it was recently determined that some of the flu shots have notable levels of mercury (part of the manufacturing process) and a weighted decision must be made on a case-by-case basis.