When i was 8, i had a mild case of whooping cough despite having been vaccinated. why did i still get it?

Whooping cough: The full potency of the vaccine might have worn off making you susceptible.

Exposure/vaccine: The primary reason you got it was a significant exposure to an infected person who transferred the germs to you.Many who have it are contagious long before it is recognized. You were also in the last wave of kids given the old whole cell vaccine, that was so harsh it was never given as a booster after 6.The vaccine we use now as a booster in older kids/adults wasn't available