Complex sentence: Such reactions are clinically benign, causing no lasting side effects or problems, but are unpleasant via a blood transfusion is estimated, as of 2006, at 1 per 2 million units transfused.
Split into simpler sentences: Such reactions are clinically benign, causing no lasting side effects or problems, but are unpleasant. Via a blood transfusion it is estimated, as of 2006, at 1 per 2 million units transfused.