They can't give the pertusssis to anyone over 7, it's not licensed for adult use.
Aw, thanks for the info.
That explains it, because I looked at his records and when we went to China he was about 8 and got a Td (that's what it was called instead of DT), and this year they gave him another booster of Td because he's going to Central America (but he also went last year, and they didn't give him a booster, so I'm supposing they have about a 10 year effectiveness.)
So both times, he would have been over 7 and that's why it wasn't a DPT.
Good thing about FR, if you don't know an answer to a question, somebody else will.