This study was conducted in patients with mild/moderate symptoms of COVID-19 and assessed clinical symptom resolution with ensitrelvir (2 dose groups; high dose and low dose), orally administered once daily for five days, compared to placebo. A total of 1,821 patients were enrolled, in Japan, South Korea, and Vietnam, irrespective of risk factors for COVID-19 progression. The majority of patients were previously vaccinated. The primary endpoint in the study was the time to resolution of five key COVID-19 symptoms (stuffy or runny nose, sore throat, cough, feeling hot or feverish, and low energy or tiredness) which are characteristic of infection with the SARS-CoV-2 Omicron variant, in patients randomized within 72 hours from the onset of symptoms. The five assessed symptoms were selected in consultation with medical experts and regulatory authorities including the Ministry of Health, Labor and Welfare (MHLW), the Pharmaceuticals and Medical Devices Agency (PMDA) in Japan and the U.S. Food and Drug Administration (FDA), based on their scientific and medical validity.