Prophylactic study for COVID-19 with Kampo medicines

INTERVENTION: Participants are randomly divided into two groups, a herbal medicine group, and a placebo group. Participants in each group continue to take 9 tablets twice a day orally before or between meals for 8 weeks (56 days). CONDITION: COVID‐19 ; Respiratory Tract Infections PRIMARY OUTCOME: The number of COVID‐19 PCR positives with symptoms SECONDARY OUTCOME: 1. The period from infection to onset; 2. The period from the appearance of symptoms to the disappearance of PCR positive; 3. Number of days until the appearance of symptoms or improvement of symptoms; 4. Severe stage: Presence of hospitalization with oxygen inhalation; 5. Shock stage: ICU management required for mechanical ventilation, shock vitals, or organ failure other than lungs INCLUSION CRITERIA: 1) Age from 20 to 75 years old (at the time of registration) 2) Asymptomatic and body temperature below 37.0 degrees Celsius (at the time of registration) 3) Capable of eating orally