This study aims to evaluate whether oral probiotic supplementation can modulate the immune response induced by COVID-19 vaccination. Specifically, the study will assess the effects of probiotics on vaccine-induced antibody responses and cellular immune responses, and explore whether probiotics may enhance or accelerate immunity and reduce vaccine-related adverse effects. A total of 140 participants are planned to be enrolled.

Large-scale clinical trials have demonstrated that COVID-19 vaccines effectively reduce the incidence of COVID-19, as well as the risk of severe disease and mortality. These protective effects are primarily mediated through vaccine-induced immune responses, including both humoral immunity (antibody production) and cellular immunity.

Probiotics are widely recognized for their potential health benefits and are commonly consumed in foods such as yogurt. Emerging evidence suggests that probiotic supplementation may enhance immune responses to vaccination, as demonstrated in studies involving influenza vaccines.

This study is designed to investigate whether oral probiotic supplementation can modulate the immune response to COVID-19 vaccination. The study will evaluate both antibody responses and cellular immune responses following vaccination, and assess whether probiotics can enhance or accelerate the development of vaccine-induced immunity. In addition, the study will explore whether probiotic supplementation may reduce vaccine-related adverse effects.

A total of 140 participants are planned to be enrolled in this study.

Inclusion Criteria:

1. Male or female subject ≥ 20 to ≤ 65 years of age at the time signing informed consent form.
2. Body weight ≥50 kgs
3. Must be scheduling to receive a BNT162b2 mRNA COVID-19 vaccine
4. Being able to comply with the study protocol
5. Being able to independently provide written informed consent

Exclusion Criteria:

1. Taking antibiotic at enrollment
2. Taking immunomodulatory (e.g., steroid) or anti-histamine drugs
3. Took pre- or probiotics within the past 30 days
4. Being to expected to take other probiotic supplement during the study period
5. Being expected to take antibiotic during the study period
6. A history of hypersensitivity or severe allergic reaction to any components of the probiotics or placebo supplement
7. Pregnant or breastfeeding
8. Having a severe immunocompromised disorder (e.g. HIV infection)
9. Solid organ or bone marrow transplant recipients
10. Major gastrointestinal surgery within 3 months before enrollment
11. Having severe active gastrointestinal conditions, including inflammatory bowel disease or diverticulitis
12. No self-reported symptoms of an upper airway infection with the past 2 weeks
13. A history of SARS-CoV-2 infection
14. Previously participated in investigational study involving SARS-CoV-2 vaccines or therapies

This is a single-blind (participant-blinded) study. Participants will be randomized to receive either probiotic capsules or matching placebo capsules. The placebo capsules contain microcrystalline cellulose and are identical in appearance, packaging, and administration schedule to the probiotic capsules. Participants will be blinded to treatment assignment, while investigators and study staff will not be blinded.