Participants in this study will be given either CDI-988 or placebo orally before receiving a norovirus challenge virus and continuing for 5 days. Participants will not know whether they are getting placebo or CDI-988. The study will evaluate how well CDI-988 compared to placebo can reduce the incidence of clinical symptoms after a challenge with norovirus. The amount of virus in stool samples will be measured over time. Side effects and pharmacokinetics (the amount of CDI-988 in blood) will also be measured.

This is a single center Phase 1b randomized, double-blind, placebo-controlled study. The primary objective will be to evaluate the efficacy of CDI-988 in comparison to placebo in reducing the incidence of clinical symptoms after challenge with norovirus. Secondary objectives will be to evaluate the efficacy of CDI-988 in comparison to placebo in reducing viral shedding and disease severity and to evaluate the safety of CDI-988 in comparison to placebo.

Inclusion Criteria:

• Healthy male or non-pregnant female
• Aged 18 to 49 years
• Good state of health
• Known fucosyl transferase 2 (FUT2) secretor status

Exclusion Criteria:

• History of participation in any norovirus challenge or vaccine clinical trial
• Receipt or planned receipt of any non-live vaccines within 7 days or live vaccines within 30 days before screening or prior to Day 28
• History of suspected norovirus gastroenteritis or chronic/recurrent gastrointestinal symptoms (e.g., diarrhea or vomiting) within 2 years prior to screening
• History of diagnosed gastrointestinal malabsorption disorders, major gastrointestinal surgery, or diagnosed chronic GI conditions
• Presence of moderate or severe illness, fever (≥38°C), or diarrhea/vomiting within 7 days prior to challenge
• Any acute illness on Day 1 (dosing day)
• Positive Day 0 stool tests for enteric pathogens
• Body weight <45 kg or BMI <18 or >32 kg/m² on Day 0
• Use of immunosuppressive therapy within 6 months prior to Day 0 or having any primary or secondary immunocompromising condition
• Use of high-dose systemic corticosteroids (≥20 mg/day prednisolone for ≥2 weeks) or high-dose inhaled steroids for >7 days within 6 months

Only aggregate participant data will be available from this study