vaccines not working???

i have 3 kids. Both of my oldest have had ALL vaccines required.  My youngest has had NONE.
Oldest 2 have received 5 DTaP, last one in 2004.  In dec 2008, there was a pertussis outbreak at their school.  My 2 vaccinated kids (protected from this) got pertussis.  My youngest did not catch it.  There where LOTS of "protected" (vaccinated) kids who got it.  Is this vaccine not working? I asked the local helth dept and they said they are coming to relize that the vaccine wears off in 2 yrs.  but everything else i've seen says they should be protected for the next 10 yrs after the 5th shot.  why would they get it?  why didn't my youngest and unprotected son get it? Why did I get my 2 oldest vaccinated for no reason? Now those 2 are protected for life, they don't need another booster, right?  
thanks,
momof3confused

[jeremiahsmommy]

Hi, my son is almost 2 and I have not given him any vaccinations.  I cannot really answer your question of why your older two but not your younger got sick, but I can give you a website with lots of vaccine info.  It is educate before you vaccinate.  Just google this and you will get tons of great facts.  Hope this helps.