Dear Hype,
my 5 year old had whooping cough last fall (not vaccinated).
I gave one dose of Pertussin 200C once I knew he had been exposed and showed some initial symptoms.
The whoop came anyway. From then on I prescribed (with the help of some very kind people on this forum) according to symptom picture. The whole thing lasted about 4 weeks altogether, with no complications.
Would I say the Pertussinum didn't protect? It didn't prevent it entirely, but then in hindsight his symptoms never got very severe (no vomiting, not too many spasmodic coughing spells - though it seemed awful at the time).
If Drosera comes into the picture, please note that Hahnemann warns not to repeat the dose too quickly in his MM Pura: "Care should be taken not to give a second dose (or any other medicine) immediately after the first dose, for that would inevitably not only prevent the good result, but do serious injury...". The waiting is especially difficult, if you don't see an improvement right away. A good way of keeping track once the cough breaks out is to note down the number of attacks the child has.

Good luck and best wishes,
Karin