A Trial to Evaluate the Effect of CD388/MK1406 on the Immunogenicity of Fluzone® HD Vaccine (CD388.SQ.1.08/MK-1406-007)

August 17, 2026 updated by: Cidara Therapeutics Inc., a subsidiary of Merck & Co., Inc. (Rahway, New Jersey USA)

A Phase 1, Double-blind, Randomized Trial to Evaluate Safety and Immunogenicity of Fluzone® High-Dose Influenza Vaccine When Concomitantly Administered With CD388, a Novel Long-Acting Antiviral Conjugate for the Prevention of Influenza

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). Researchers are looking for other ways to prevent severe illness from the influenza virus.

The goals of this study are to learn if:

  • MK-1406 (formerly CD388) is safe to take with Fluzone®
  • MK-1406 affects the immune response to Fluzone®

Study Overview

Status

Active, not recruiting

Conditions

Intervention / Treatment

Detailed Description

This is a Phase 1, double-blind, randomized trial to evaluate the immunogenicity of Fluzone® HD influenza vaccine when concomitantly administered with either MK-1406 or placebo, in healthy participants. This study will also evaluate the safety and tolerability of MK-1406 when administered with Fluzone® High-Dose (HD) compared to Fluzone® HD with placebo.

Study Type

Interventional

Enrollment (Estimated)

710

Phase

  • Phase 1

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

  • United States
    • California
      • Glendale, California, United States, 91206
        • California Clinical Trials Medical Group (CCTMG) managed by Parexel
      • Glendale, California, United States, 91206
        • Parexel ( Site 0001)
    • Maryland
      • Baltimore, Maryland, United States, 21225
        • Parexel International - EPCU Baltimore
      • Baltimore, Maryland, United States, 21225
        • Parexel ( Site 0002)

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Adult

Accepts Healthy Volunteers

Yes

Description

Inclusion Criteria:

The main inclusion criteria include but are not limited to the following:

  • Be ≥18 to ≤49 years of age
  • Be deemed healthy by the Investigator
  • Have not received any seasonal influenza vaccine and have not had a diagnosed or suspected influenza infection within 12 months prior to Day 1 of the trial

Exclusion Criteria:

The main exclusion criteria include but are not limited to the following:

  • Had an active malignancy within 5 years prior to screening, except basal cell or squamous cell skin cancer. Any history of breast cancer or melanoma is exclusionary.
  • Has previously been diagnosed with Guillain-Barre Syndrome following a previous influenza vaccination.

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Prevention
  • Allocation: Randomized
  • Interventional Model: Parallel Assignment
  • Masking: Quadruple

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Fluzone® High-Dose (HD) plus MK-1406
Participants will receive open-label Fluzone® HD influenza vaccine by intramuscular (IM) injection plus MK-1406 by subcutaneous (SC) injection on Day 1.
Biological: Fluzone® High-Dose (HD) influenza vaccine
Fluzone® HD injectable suspension administered by intramuscular (IM) injection
Other Names:
  • Fluzone® High-Dose Influenza Vaccine
Combination product: MK-1406 Injection
MK-1406 liquid for injection administered subcutaneously
Other Names:
  • CD388
Placebo Comparator: Fluzone® HD plus Placebo
Participants will receive open-label Fluzone® HD influenza vaccine by IM injection plus placebo by SC injection on Day 1.
Biological: Fluzone® High-Dose (HD) influenza vaccine
Fluzone® HD injectable suspension administered by intramuscular (IM) injection
Other Names:
  • Fluzone® High-Dose Influenza Vaccine
Combination product: Placebo
MK-1406 matched liquid for SC injection

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Geometric Mean of Hemagglutinin Inhibition (HAI) Titers Against the Influenza Strains Contained in the Vaccine Among Participants Receiving Concomitant Fluzone® High-Dose (HD) and MK-1406 Compared to Participants Who Receive Fluzone® HD and Placebo
Time Frame: Predose (Day 1), Day 15 and Day 29
Geometric mean of HAI titers at 2-weeks and 4-weeks post-trial intervention against the influenza virus strains contained in the vaccine among participants receiving concomitant Fluzone® HD influenza vaccine and MK-1406 compared to participants who receive Fluzone® HD and placebo will be determined. HAI testing will be performed using validated methods in accordance with applicable Good Clinical Laboratory Practice (GCLP) and laboratory Standard Operating Procedures (SOPs). Geometric Mean Titers (GMTs) will be summarized by treatment group and influenza strain.
Predose (Day 1), Day 15 and Day 29
Percentage of Participants with Seroconversion Rates against the Influenza Strains Contained in the Vaccine Among Participants Receiving Concomitant Fluzone® HD and MK-1406 Compared to Participants Who Receive Fluzone® HD and Placebo
Time Frame: Predose (Day 1), Day 15 and Day 29
Seroconversion is defined as a 4-fold increase in influenza titer as measured by HAI assay at each visit compared to baseline. Seroconversion rates at 2-weeks and 4-weeks post-trial intervention against the influenza virus strains contained in the vaccine among participants receiving concomitant Fluzone® HD influenza vaccine and MK-1406 compared to participants who receive Fluzone® HD and placebo will be determined. Seroconversion rates will be summarized by treatment group and influenza strain.
Predose (Day 1), Day 15 and Day 29

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Percentage of participants with ≥1 Adverse Event (AE)
Time Frame: Up to approximately Day 29
An AE is defined as any untoward medical occurrence in a participant administered a pharmaceutical product temporally associated with the use of study treatment, whether or not considered related to the study treatment.
Up to approximately Day 29
Percentage of Participants Who Discontinued from the Study Due to an Adverse Event
Time Frame: Up to approximately Day 29
An AE is defined as any untoward medical occurrence in a participant administered a pharmaceutical product temporally associated with the use of study treatment, whether or not considered related to the study treatment. The percentage of participants who discontinued the study because of an AE (as defined above) will be assessed.
Up to approximately Day 29
Percentage of Participants with a Solicited Injection-site Adverse Event
Time Frame: Up to approximately Day 8 post-dose
An AE is any untoward medical occurrence in a patient or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. The solicited injection-site AEs assessed are redness/erythema, swelling, and tenderness/pain.
Up to approximately Day 8 post-dose
Percentage of Participants with a Solicited Systemic Adverse Event
Time Frame: Up to approximately Day 29
An AE is any untoward medical occurrence in a patient or clinical study participant, temporally associated with the use of study treatment, whether or not considered related to the study treatment. The solicited systemic AEs assessed are muscle pain/myalgia, joint pain/arthralgia, headache, and tiredness/fatigue.
Up to approximately Day 29

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Cidara Therapeutics Inc., a subsidiary of Merck & Co., Inc. (Rahway, New Jersey USA)

Investigators

  • Study Director: Medical Director, Merck Sharp & Dohme LLC

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

November 8, 2025

Primary Completion (Estimated)

December 31, 2026

Study Completion (Estimated)

December 31, 2026

Study Registration Dates

First Submitted

December 10, 2025

First Submitted That Met QC Criteria

December 10, 2025

First Posted (Actual)

December 23, 2025

Study Record Updates

Last Update Posted (Actual)

August 19, 2026

Last Update Submitted That Met QC Criteria

August 17, 2026

Last Verified

August 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

  • CD388.SQ.1.08 (Other Identifier: Cidara)
  • MK-1406-007 (Other Identifier: MSD)

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

YES

IPD Plan Description

https://trialstransparency.msdclinicaltrials.com/pdf/ProcedureAccessClinicalTrialData.pdf

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

Yes

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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