This is the major issue with this vaccine. While the Td and DT vaccines rarely cause more than a sore arm, the Tdap and DTaP vaccines have many more side effects. The additional side effects are presumably from the pertussis component of the vaccines.
For the DTaP vaccine, the Vaccine Information Statement (VIS) has a long list of possible side effects. These start with mild reactions: fever, redness, swelling, tenderness, and others in up to one in three recipients. The next level of reactions are called “moderate”: nonstop crying for more than three hours in one child in a thousand, seizures in one child in fourteen thousand, high fever over 105ºF in one child in sixteen thousand. The final level of reactions is classified as “severe”: a severe allergic reaction, long-term seizures, coma, lowered consciousness, and permanent brain damage. While the severe allergic reaction is listed as occurring in less than one out of a million doses, the other severe reactions are listed as so rare that experts are not sure if they are caused by the vaccine. However, the moderate reactions are real and scary enough.
The Tdap vaccine has fewer side effects listed, and most of them are similar to the mild reactions previously covered: pain, redness, fever, plus some nausea, vomiting, and headaches. None were reported as permanent but interestingly enough, they occurred more often than with the DTaP vaccine, in up to 75 percent of the recipients in some cases.