Last week my fourth child (completely unvaxed) came down with whooping cough. It is not a "pain in the hiney" or a "nuisance" it is HORRIBLE. There were a few days I was about to lose my mind I was so afraid he'd pass out or die in my arms in the night There is no possible way you can understand how bad it is until you see YOUR child not breath, or cough until he's bright red, tears streaming ... and then start again. My son never even turned blue or vommitted (thank you Lord) but it was still awful, awful, awful.
Again, no one can understand unless they've been there.
and trust me, you dont want to be.
My son had it at two years (partially vaxed and nursing often) and passed it to me who was 5.5 months pregnant and fully up to date on the vax (military). I've had pneumonia before this as a college student and a mean bout of bronchitis since and pertussis was a unique experience--I had every symptom in the book including the vomiting.
It is a risk whichever choice you make. I viewed the risk from a vaccine (with limited efficacy) that I allowed to be deliberately injected, to be greater than the likelihood that my children would just happen to contract a serious case of pertussis; and for us this was true. I attacted every sniffle and cough with sodium ascorbate and their only two significant illnesses (which the whole family caught) was some beastly respiratory illness when they were 3 and 5, and another a year later resulting from ds' pinkeye infection--he coughed until he was bright red and tears streaming; it's the only time we took him to the doctor for a cough.
In any case, your child is old enough that I would not be as concerned about a reaction from the vaccine; I understand that Dr. Sears' book has information about immune support (vitamins and such) when vaccinating.