My daughter was at playgroup and collided with an older and bigger girl. She was knocked down backwards. She cried but not too much and then played. After about 15 minutes she was not acting like herself, wanting to leave early and wimpering. My husband was with her and theyleft. After about 15 more minutes she vomited. She was starting to go down fast. He took her to the ER and they said go see the ped right now. While he was waiting in the room he gave her the shot as she was getting more lethargic and not talking. So, I guess this was aobut 30 minutes. But I was shocked that such a seemingly little thing set it all off. This has been her only injury but she has need the shot for vomiting and high fevers. We play it safe. Julia