A Study to Evaluate the Safety and Efficacy of MK-1406/CD388 for Prevention of Influenza (MK-1406-005/CD388.SQ.3.06)

Approximately one billion cases of seasonal influenza occur annually. Of these, 3 to 5 million illnesses are severe and responsible for up to 650,000 deaths per year (WHO 2025). Yearly administration of an influenza vaccine for the prevention of influenza is currently recommended. However, the real-world vaccine effectiveness varied from 10% to 60% in the general population across the years of 2004 to 2024, with effectiveness in most years below 50% (CDC 2025) and decreasing to as low as 5% in immunocompromised individuals (Hughes 2021). The goal of this study is to learn whether MK-1406 (CD388) can help prevent the flu in people who are at higher risk of becoming seriously ill from influenza. Researchers want to find out if MK-1406 dosed at a single study visit can provide protection against influenza compared with placebo. The study will include adolescents and adults who may be more vulnerable to complications from influenza because of their age, health conditions, or weakened immune systems. Researchers will also evaluate the safety of MK-1406 and how well participants tolerate the study medicine.

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: Primary Stratum A (non-immunocompromised participants who have chronic stable medical conditions) and Primary Stratum B (participants who are immunocompromised either due to underlying disease or receipt of immunosuppressive medications) * Has negative rapid antigen tests for influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prior to dosing at Day 1 * Weight is ≥ 40 kg Primary Stratum A only * Has a history of pulmonary disease * Has moderate to severe asthma, as defined by the Global Initiative for Asthma (GINA 2025) * Has existing cardiac disease * Has insulin-dependent diabetes * Has moderate renal impairment * Is ≥ 65 years of age at the time of randomization but does not meet any of the above criteria for Primary Stratum A, and is otherwise healthy Primary Stratum B only * Has a solid tumor diagnosis AND has received chemotherapy and/or immunotherapy within 1 year of screening * Has a diagnosis of a hematologic malignancy within 5 years of screening AND has received any chemotherapy or biologic therapy within 1 year of screening * Participants who have had a solid organ transplant (SOT) must satisfy all of the following: * Has received a kidney, liver, heart, or lung transplant more than 6 months prior to screening * Is currently receiving at least two immunosuppressive medications * Participants who have had a hematopoietic stem cell transplant (HSCT) must satisfy at least one of the following: * Has a history of hematopoietic stem cell transplantation (HSCT) (i.e., autologous, allogeneic, bone marrow, peripheral blood stem cell, tandem \[peripheral blood and marrow\]) within 1 year of screening * Has a history of non-autologous HSCT with graft-versus-host disease (GvHD) requiring active treatment with immunosuppressants (e.g., systemic corticosteroids ≥1 mg/kg at screening), regardless of the duration of time since HSCT * Is receiving immunosuppressive medicines * Has received chimeric antigen receptor-modified T-cell therapy * Has received B-cell depleting therapies (e.g., rituximab, ocrelizumab, ofatumumab, alemtuzumab) within the 12 months prior to screening * Has a diagnosis of any primary immunodeficiency except immunoglobulin A (IgA) deficiency * Has advanced or untreated human immunodeficiency virus (HIV) infection Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Known or suspected allergy or history of anaphylaxis or other serious adverse reactions to zanamivir (following administration of inhaled or intravenous formulations), monoclonal antibody (mAbs (including Fc domains)), or any of the components of CD388 or placebo * Has an acute (time-limited) or febrile (temperature ≥ 38.0°C \[≥ 100.4°F\]) illness within 7 days prior to planned dosing on Day 1 * Has severe chronic kidney disease (CKD) or is receiving hemodialysis * Receipt within the past 30 days or 5 half-lives (whichever is longer) or anticipated receipt of any drug or other biologic agent (e.g., mAbs) administered for the prevention or treatment of influenza * Has a clinically significant bleeding disorder (e.g., factor deficiency, coagulopathy, or platelet disorder) or medical history of significant bleeding or bruising following intramuscular or subcutaneous (SC) injections or venipuncture * Has previously enrolled in this study (CD388.SQ.3.06)