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A Phase 3 Study of Efitazanamivir Alfa for Preventing Influenza in Adults and Adolescents at High Risk of Complications

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What is this trial about?

A plain-language summary of the goals, design and what participants do

The study focuses on influenza, a viral infection that can cause fever, cough, and body aches, and targets adults and adolescents who have a higher chance of serious complications, such as people over 65, those with stable chronic health problems, and individuals with weakened immune systems. Participants receive either the investigational drug CD388, which contains the active substance efitazanamivir alfa and is administered by subcutaneous injection, or a matching placebo that looks the same but contains no active medication.

The purpose of the trial is to determine whether a single dose of the study drug can prevent symptomatic, laboratory‑confirmed influenza compared with placebo. After the injection, participants are monitored for up to 24 weeks for any signs of flu‑like illness; if symptoms appear, a nasal swab is tested using RT-PCR (a laboratory method that looks for the virus’s genetic material) to confirm infection. Throughout the follow‑up period, researchers also record any adverse events, check the injection site, and assess vital signs and routine lab results to evaluate safety and tolerability.

The research process

The trial runs in 7 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline Visit and Randomization

    You will attend a study visit where your health status is confirmed and you will give informed consent to take part in the trial.

    During this visit you will be randomly assigned to receive either MK-1406 or a matching placebo. The assignment is double‑blind, meaning you and the study staff will not know which product you receive.

  2. Step 2

    Study Injection

    On the same day as the baseline visit, you will receive a single subcutaneous injection (injection under the skin).

    If you are assigned to the active arm, the injection contains efitazanamivir alfa at a dose of 450 mg in a solution for injection. If you are assigned to the placebo arm, the injection contains a matching solution without active drug.

    The injection is administered by qualified staff and takes only a few minutes.

  3. Step 3

    Observation Period (First 7 Days)

    For the first seven days after the injection you will be asked to monitor yourself for any immediate reactions at the injection site, such as redness, swelling, or pain, and to report any other symptoms.

    You may be contacted by the study team to record vital signs (blood pressure, pulse) and to ask about any adverse events (AEs), which are any unwanted medical occurrences.

  4. Step 4

    Regular Follow‑Up Visits

    You will return for scheduled follow‑up visits approximately every four weeks until week 24 (about six months).

    At each visit the study staff will check your vital signs, collect blood samples to measure the level of the study drug in your blood, and ask about any adverse events or injection‑site reactions.

    These visits help to assess the safety of the product over the entire study period.

  5. Step 5

    Monitoring for Influenza‑Like Illness

    From day 7 after the injection through week 24 you will be instructed to report any symptoms that could indicate influenza‑like illness (ILI).

    ILI is defined as a confirmed influenza infection by a laboratory test called RT‑PCR (a test that detects the virus’s genetic material) together with new or worsening respiratory symptoms (such as cough or shortness of breath) and at least one systemic symptom (such as fever or muscle aches).

    If you develop such symptoms, a nasopharyngeal swab (a gentle throat‑nose sample) will be taken for the RT‑PCR test.

  6. Step 6

    Safety Assessments

    Throughout the 24‑week period the study will collect information on any adverse events, injection‑site reactions, and changes in laboratory test results.

    The study will also record any hospitalizations or deaths that occur within 30 days after a confirmed influenza infection, although these events are expected to be rare.

  7. Step 7

    Final Study Visit (Week 24)

    At the end of the 24‑week follow‑up you will attend a final visit where the study team will perform a last set of safety evaluations, collect a final blood sample, and review any remaining health concerns.

    After this visit the trial participation is completed.

Who can join the trial?

28 criteria

  • Be at least 12 years old when you sign the consent form.
  • Provide a signed informed consent (or have a parent/guardian sign if you are under the legal age) and any required authorizations.
  • Test negative for both influenza (flu) and COVID‑19 (SARS‑CoV‑2) using a rapid antigen test before receiving the study medication on Day 1.
  • Weigh at least 40 kg (about 88 lb) at the screening visit.
  • Have a Body Mass Index (BMI) of 18 kg/m² or higher at screening (BMI is a simple calculation that compares your weight to your height).
  • Be able to follow the study procedures, read and complete electronic diary questions on a smartphone, tablet, or computer, and understand the language used in the diary.
  • Be medically stable, with no major changes in your regular medicines and no hospital stay for a worsening condition in the past two months.
  • If you are female, agree to use birth‑control methods as described in the study protocol.
  • Agree not to donate blood or blood products from the first day of the study until 32 weeks after the study medication is given.
  • Be willing to provide a valid form of identification and be reachable for study communication.
  • Primary Stratum A – Lung disease: Have a history of lung problems such as chronic obstructive pulmonary disease (COPD, a condition that makes breathing hard), bronchiectasis, cystic fibrosis, interstitial lung disease, pneumoconiosis, or past bronchopulmonary dysplasia.
  • Primary Stratum A – Asthma: Have moderate to severe asthma (a condition causing wheezing and shortness of breath) as defined by current treatment steps.
  • Primary Stratum A – Heart disease: Have one of the following heart conditions:
    • Congenital heart disease (a heart problem present from birth).
    • Chronic heart failure classified as NYHA Class II‑IV (a scale doctors use to describe how much heart failure limits activity).
    • Coronary artery disease that has been stable for at least six months and requires regular medication or follow‑up.
    • High blood pressure with heart complications such as heart failure, irregular heartbeat (e.g., atrial fibrillation), enlarged heart, or reduced heart function.
    • Primary Stratum A – Diabetes: Have diabetes that requires insulin injections.
    • Primary Stratum A – Kidney function: Have moderate kidney impairment (Stage 3 chronic kidney disease), meaning your kidneys work at 30‑59 mL/min/1.73 m² as measured by a standard calculation.
    • Primary Stratum A – Age 65+: Be 65 years or older at randomization, otherwise healthy, and not meeting any of the other A‑criteria.
    • Primary Stratum B – Solid tumor: Have a solid tumor (cancer) and have received chemotherapy or immunotherapy within the past year.
    • Primary Stratum B – Blood cancer: Have a blood‑cancer diagnosis (hematologic malignancy) within the past five years and have received chemotherapy or biologic therapy within the past year.
    • Primary Stratum B – Solid organ transplant: Have received a kidney, liver, heart, or lung transplant more than six months ago and are currently taking at least two medicines that suppress the immune system.
    • Primary Stratum B – Stem‑cell transplant: Have had a hematopoietic stem‑cell transplant (HSCT) within the past year, or have a non‑autologous HSCT with graft‑versus‑host disease that requires immune‑suppressing treatment.
    • Primary Stratum B – Immunosuppressive medicines: Are currently taking immune‑suppressing drugs such as high‑dose steroids (≥20 mg prednisone daily), certain chemotherapy agents, or other biologic agents used for cancer or rheumatic diseases.
    • Primary Stratum B – CAR‑T cell therapy: Have received chimeric antigen receptor (CAR‑T) cell therapy, a specialized cancer treatment that modifies your T‑cells.
    • Primary Stratum B – B‑cell‑depleting therapies: Have received treatments that reduce B‑cells (e.g., rituximab, ocrelizumab, ofatumumab, alemtuzumab) within the past 12 months.
    • Primary Stratum B – Primary immunodeficiency: Have a primary immunodeficiency disorder (a condition where the immune system is weakened) except for IgA deficiency.
    • Primary Stratum B – Advanced HIV: Have untreated or advanced HIV infection with a CD4 cell count less than 350 cells/mm³ within the past six months (CD4 count is a lab measure of immune health).

Who cannot join the trial?

20 criteria

  • You are a woman who is pregnant or lactating (breast‑feeding a baby).
  • You have a known or suspected allergy, or have ever had a serious reaction called anaphylaxis, to zanamivir, monoclonal antibodies, or any ingredient in the study drug or placebo.
  • You have been diagnosed with influenza (the flu) within the past 6 months.
  • You received the seasonal flu vaccine (either the inactivated shot or the live nasal spray) within 14 days before the first study dose.
  • You have an acute illness or a fever of 38 °C (100.4 °F) or higher within 7 days before the first dose.
  • You have been in close contact (including living in the same household) with someone who has confirmed flu or SARS‑CoV‑2 (COVID‑19), or with someone treated with antiviral medicines for these infections, within the 7 days before joining the study.
  • You have a clinically unstable condition, such as a serious psychiatric problem, recent thoughts of suicide, or any illness that could lead to hospitalization or death during the study period.
  • You have a history of alcohol or drug substance abuse that, in the investigator’s opinion, could affect how the study is conducted.
  • You had major surgery (for example, heart, lung, or abdominal surgery) within 4 weeks before randomization, have not fully recovered from such surgery, or plan to have major surgery while you are expected to be in the study.
  • You have severe chronic kidney disease (CKD) (kidney function measured as eGFR less than 30 mL/min/1.73 m²) or you are receiving hemodialysis.
  • Your screening electrocardiogram (ECG) shows a prolonged QT interval (greater than 450 ms in men or 470 ms in women), a prolonged PR interval (greater than 220 ms), second‑ or third‑degree heart block, or other important rhythm problems, unless you have a known stable heart condition that explains these findings.
  • You are currently participating in, or plan to join, another clinical study that gives a study drug during the same time period.
  • You received any drug or biologic (such as monoclonal antibodies) intended to prevent or treat flu within the past 30 days (or five half‑lives of that drug), or you plan to receive one.
  • You received any form of immunoglobulin (antibody preparation) within 14 days before the planned study dose.
  • You received any experimental drug, vaccine, or biologic within 90 days (or five half‑lives) before the study dose.
  • You have a clinically significant bleeding disorder (such as a factor deficiency, coagulopathy, or platelet problem) or a history of notable bleeding or bruising after shots or blood draws.
  • You donated a large amount of blood (at least 450 mL, about one unit) within 30 days before screening.
  • You have a direct role in this study or other studies (as an investigator, study staff, family member of staff, sponsor employee), or you have previously taken part in a CD388 trial.
  • In the investigator’s judgment, you have any condition or situation that greatly increases risk, makes participation difficult, could affect the study results, or suggests you might not follow the study requirements.
  • You have already enrolled in this same study before.
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Investigated drugs

MK-1406 is an experimental, long‑acting antiviral medicine that is given as a small injection under the skin. It contains the active ingredient efitazanamivir alfa, which works to stop the flu virus from multiplying. In this study, participants receive the injection to see if it can protect them from getting flu symptoms when they are exposed to the virus, especially those who are at higher risk for serious flu complications.

What is already known about the treatment

EFITAZANAMIVIR ALFA - It is given as a 450 mg solution for subcutaneous injection, meaning it is injected under the skin. The drug is still investigational and is being studied in Phase 3 trials as a long‑acting antiviral to prevent flu. It is intended for adults and adolescents who have higher risk of flu complications, such as older adults or those with chronic health problems. The medicine works by attaching to the flu virus and stopping it from spreading, placing it in the antiviral drug class.

Investigated diseases

Influenza - Influenza is a contagious viral infection that affects the nose, throat, and lungs. It usually begins with a sudden fever, chills, muscle aches, and a dry cough. Over several days the symptoms may spread to cause a sore throat, headache, and feeling very tired. In most people the illness improves within a week, but the virus can continue to irritate the airways for a longer period. Some individuals, especially older adults or those with chronic health conditions, may experience more intense symptoms that affect breathing. The infection can move from the upper airway to the lower airway, leading to increased coughing and difficulty breathing.
Trial detailsLast updated 17 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2026-527531-33-00Protocol codeCD388.SQ.3.06Estimated enrolment10 000 patients

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