Adults don't always whoop because they have much wider airways, so when they draw in a big breath, you don't hear the sound. In children's it's caused by the air whistling because it is being drawin into the airways so quickly. Very little babies often don't whoop either because they just can't breathe in fast enough to cause the sound.

The pertussis vaccine is about 90% effective, and lasts for about 5 years. Unless you have had a booster as an adult, you are probably susceptible to the pertussis virus. And because of it's limited effectiveness, one in 10 children will still have no immunity to it either. That is one of the reasons why we still have regular outbreaks of the disease - even in a perfect world where every child was immunised with no side effects, there would still be 10% of the population who could catch the disease.