I was fully vaccinated against whooping cough yet got it when i was 8. it wasn't as severe but it was positive for pertussis. cause?

Immunity wanes: Pertussis immunity wanes-decreases- even when fully vaccinated. Protection against pertussis in childhood is highest in the first year after 5th vaccine. Although immunity may wane, those who contract pertussis will have less severe disease. As an adult one needs at least one Dtap booster. Women need a Dtap booster with each pregnancy to protect their infants.

It happens: At a listed age of 31, your Pertussis vaccines in childhood were the old whole cell vaccine that was never given after age 6 because of severe reactions. By your late teens your pertussis immunity waned enough you could get the disease. Boosters using the new acellular vaccine were not routinely given to teens or adults until relatively recently.