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Thanks for your question.
It should have started showing some effect.It should have improved to some extent at least.
Prednisone is a strong anti inflammatory and the dose you have taken is adequate.
If the rash and itching have not improved you need to show it to a doctor as there may be something else causing the rash.That needs to be confirmed.
If this is an allergic reaction and prednisone has not shown any effect you may require injection of steroids.
You may add an anti allergic like benadryl to prednisone but prednisone not having an effect till now indicates that you need to have the rash examined by a doctor.