- recent infection with coronavirus disease-2019 (covid-19) (newly positive pcr or antigen test within prior 4 months) - prior receipt of antibody therapies (convalescent or monoclonal) towards covid-19 - prior receipt of vaccines against covid-19 - prior or current participation in a clinical trial of vaccines against covid-19 - scheduled to receive a vaccine against covid-19 in the following month - regular use of any live microbial (probiotic) supplements - any acute or chronic respiratory tract disease besides mild to moderate asthma - any diagnosed immunodeficiency - current use of immunosuppressive drugs - any diagnosis of chronic gut disease, such as inflammatory bowel disease or irritable bowel syndrome - pregnancy or breastfeeding - recent use of antibiotics - current addiction to alcohol, drugs, or medications

- recent infection with coronavirus disease-2019 (covid-19) (newly positive pcr or antigen test within prior 4 months) - prior receipt of antibody therapies (convalescent or monoclonal) towards covid-19 - prior receipt of vaccines against covid-19 - prior or current participation in a clinical trial of vaccines against covid-19 - scheduled to receive a vaccine against covid-19 in the following month - regular use of any live microbial (probiotic) supplements - any acute or chronic respiratory tract disease besides mild to moderate asthma - any diagnosed immunodeficiency - current use of immunosuppressive drugs - any diagnosis of chronic gut disease, such as inflammatory bowel disease or irritable bowel syndrome - pregnancy or breastfeeding - recent use of antibiotics - current addiction to alcohol, drugs, or medications