Respiratory syncytial virus (RSV) is recognized worldwide as a significant pathogen causing lower respiratory tract infection in infants, the elderly, and immunocompromised patients, which may cause bronchiolitis or pneumonia, and affect the cardiovascular system, central nervous system and the renal system, resulting in hospitalization and death risks in children and also high-risk adults, particularly the elderly. The purpose of this study is to evaluate the efficacy of Ziresovir (AK0529) compared to placebo with respect to the time to resolution of RSV lower respiratory tract disease (LRTD) symptoms.