system
Given the following post, respond as if you're commenting on Reddit:
human
Children who experience severe reactions to the TDaP vaccine receive the Td vaccine instead. What is it about the addition of the pertussis protection that makes people react?

Disclaimer: I AM EXTREMELY PRO-VACCINE. I know that these reactions are unbelievably rare. 

How come people respond differently/aren’t “allergic” (if that’s even the mechanism behind the reactions?) to the Td when they are to the DTaP? 

I’ve tried to research on my own but am worried I’m accidentally finding antivaxx nonsense studies that lack valid data.
gpt
That is not about all adverse reactions. It's specifically about seizure and encephalopathy. 

There appears to be a higher risk of seizure in the short time after Tdap, that is not present after Td. Seizures are rare, so the numbers are small and this effect may not really exist, but we treat it as real until proven otherwise.

If someone has had seizure after Tdap, they should in the future receive Td instead, unless evaluation can attribute the seizure to something else.

On the other hand if someone has a severe *allergic* reaction to Tdap, they should not receive any vaccine containing any of the components, unless they undergo allergy testing to see which was the trigger. 

Note that this is not a contraindication to receiving Tdap or any other vaccine. If a person stays unvaccinated and develops pertussis, the infection is even more likely cause seizures, and may even trigger the development of lifelong seizure disorder.