Effect of Surgical Masks in Preventing Acute Respiratory Infections

Inclusion Criteria: * Those who originally do not wear a mask during the influenza season * Those aged 20 years or older and under 50 years at the time of providing informed consent * If assigned to the mask group, those who can agree to wear a mask when going outdoors for 3 to 4 months during the influenza season (excluding times such as when eating) * In the case of the non-mask group, those who can agree, in principle, not to wear a mask for 3 to 4 months during the influenza season * Those who can agree to undergo testing for both influenza and COVID-19 on the day of onset or the following day if symptoms suggestive of influenza, COVID-19, or a common cold (such as fever or respiratory symptoms) develop, and to visit a medical institution at their own expense for medical fees * Those who usually go out 3 or more days a week * Those who have obtained the agreement of cohabiting family members regarding participation * Those who, after receiving a thorough explanation regarding participation in this study and with full understanding, have provided written informed consent based on their own free will Exclusion Criteria: * Those who originally wear a mask during the influenza season and intend to continue doing so in the future * Those who are engaged in healthcare, food manufacturing, etc., and wear a mask during regular work * Those who are pregnant * Those with underlying medical conditions such as chronic liver disease, cancer, or cardiovascular disease * Those who have previously experienced allergic symptoms when wearing commercially available non-woven masks * Those who are planning long-term overseas travel exceeding one week during the influenza season, including the New Year holidays‡ * Others who did not provide consent