Inclusion Criteria:  Healthy adults (any gender, ages 18 to 60 years old) in good general health, based on medical history SARS-CoV-2 infection >4 months prior confirmed by a positive polymerase chain reaction (PCR) or antigen test Body mass index (BMI) of 18.5-39.9 kg/m2 Agreement to comply with the protocol and study restrictions Access to internet in addition to willingness and ability to use web-based questionnaires Available for all study visits English-speaking  Exclusion Criteria:  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