The risk is very very rare.
From the CDC:
"Mild Problems (Common)
• Fever (up to about 1 child in 4)
• Redness or swelling where the shot was given (up to
about 1 child in 4)
• Soreness or tenderness where the shot was given (up
to about 1 child in 4)
These problems occur more often after the 4th and 5th
doses of the DTaP series than after earlier doses.
Sometimes the 4th or 5th dose of DTaP vaccine is
followed by swelling of the entire arm or leg in which
the shot was given, lasting 1-7 days (up to about 1
child in 30).
Other mild problems include:
• Fussiness (up to about 1 child in 3)
• Tiredness or poor appetite (up to about 1 child in 10)
• Vomiting (up to about 1 child in 50)
These problems generally occur 1-3 days after the shot.
Moderate Problems (Uncommon)
• Seizure (jerking or staring) (about 1 child out of
14,000)
• Non-stop crying, for 3 hours or more (up to about
1 child out of 1,000)
• High fever, over 105oF (about 1 child out of
16,000)
Severe Problems (Very Rare)
• Serious allergic reaction (less than 1 out of a million
doses)
• Several other severe problems have been reported
after DTaP vaccine. These include:
- Long-term seizures, coma, or lowered consciousness
- Permanent brain damage.
These are so rare it is hard to tell if they are caused
by the vaccine.
That's what my pediatrician told me, when my son was small.
Like I said, that was 17 years ago, and we had the pertussis with the DT but I can't tell you the number of people I know who opted out of that vaccine.
Now, as to a booster DPT, my son just had shots to go on a trip out of the country. The county health department gave him a booster DT, it did not contain "P".
You'd think since this is going on, county health departments would be alerted and when they had the chance, if a booster was needed, would include the pertussis.