I had Whooping Cough when I was 17 and I was immunised. Orginially I went to the Dr with this bad cough and he just said it was the back of my nose dripping onto my throat causing me to cough. I highly doubted this as I'm not someone who is prone to coughs etc.

Anyway about 2 months went past and by this stage my younger brother (11) also immunised had the cough. He went to the Dr, by this stage there was a bit more in the media about Whooping Cough being around, he tested my brother and then of course decided to test me. And I had WC, had had it for around 2 months, if I wasn't immunised I would have obviously been a lot sicker than what I was.

So in short I think for you DS, maybe being a bit more precausious might be a good thing, especially given his age. Hopefully though it is nothing more than a virus.