This pilot double-blind randomized controlled trial will test the preliminary efficacy of two doses of a combination of live microbials (probiotics) given to boost the immunity of unvaccinated persons previously infected with SARS-CoV-2.

This pilot double-blind randomized controlled trial will test the preliminary efficacy of two doses (a standard dose and a high dose) of a combination of live microbials (probiotics), currently marketed in food (including products labeled as dietary supplements), given daily for 21 days to boost the immunity of unvaccinated, generally healthy persons ages 18-60 with prior confirmed SARS-CoV-2 infection. Data collected will include demographics, comorbidities, medications, information about prior SARS-CoV-2 infection, and self-reported symptoms and adverse events (AEs). Biospecimens (saliva, nasal wash, blood, and stool) will be collected at baseline and days 21 and 42 for analysis.

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

Saliva, nasal rinse, blood samples, and stool samples collected will be used for the purposes of this research.

After information that could identify consenting participants has been removed, de-identified information and left over biospecimens collected for this research may be used by or distributed to the study funder (Danisco USA Inc.) or to investigators for other research without obtaining additional informed consent. Personal identifying details, such as name and date of birth, will not be shared without participants' additional permission.

Data will be shared after enrollment and data analysis are completed.