I have to disagree. Vaccines in general don't just "lessen the symptoms if you catch it" - they're designed to prevent you from catching it in the first place
. That's the whole purpose of them. And if you are unlucky enough to catch it anyway, then yes, you are correct in that you may acquire a lesser form of the disease. But saying that vaccines don't prevent disease at all is entirely misleading.
Of course they don't have a 100% prevention rate - nothing does - but the pertussis vaccine has quite good protection rates: "A 3-dose primary series of immunisation with DTPa [diphtheria, pertussis and tetanus vaccine in Australia] at 2, 4 and 6 months of age results in 84–89% protective efficacy against disease