To prevent acute rheumatic fever, acute pharyngitis due to group A streptococci should be treated with antibiotics before:

c. 9 days of illness(Ref: Nelson's 20/e p 2269-2271)Antibiotic therapy for patients with GAS pharyngitis can prevent acute rheumatic fever, shorten the clinical course of the illness, reduce transmission of the infection to others, and prevent suppurative complications.When therapy is instituted within 9 days of onset of symptoms and continued for the full course, acute rheumatic fever is almost always prevented.