I really doubt that what you are seeing is a thiamine deficiency. I could be wrong. Is your chick eating any food? I think it can take 3 weeks for a thiamine deficiency to affect a chicken. I would continue giving the Corid plus a daily boost of undiluted Corid, and if you lose one more, take or send it to your state vet for a necropsy. Coccidiosis can cause weakness and dehydration, and can sometimes look like other illnesses. Sorry for your loss.