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

NCT07300085 | Active Not Recruiting Phase 1 FDA Drug

• 2 other identifiers: CD388.SQ.1.08MK-1406-007
• Study Type: interventional
• Target: 710
• Locations: 1 country
• Sites: 4

Brief Summary

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®

Conditions

Healthy Participants

Keywords

Influenza; Antiviral Agents

Outcome Measures

Primary Outcomes (2)

• 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

  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

  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 Outcomes (4)

• Percentage of participants with ≥1 Adverse Event (AE)

  Up to approximately Day 29

• Percentage of Participants Who Discontinued from the Study Due to an Adverse Event

  Up to approximately Day 29

• Percentage of Participants with a Solicited Injection-site Adverse Event

  Up to approximately Day 8 post-dose

• Percentage of Participants with a Solicited Systemic Adverse Event

  Up to approximately Day 29

Study Arms (2)

Fluzone® High-Dose (HD) plus MK-1406

EXPERIMENTAL

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; Combination Product: MK-1406 Injection

Fluzone® HD plus Placebo

PLACEBO COMPARATOR

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; Combination Product: Placebo

Interventions

Placebo COMBINATION_PRODUCT

MK-1406 matched liquid for SC injection

Fluzone® HD plus Placebo

Fluzone® High-Dose (HD) influenza vaccine BIOLOGICAL

Fluzone® HD injectable suspension administered by intramuscular (IM) injection

Also known as: Fluzone® High-Dose Influenza Vaccine

Fluzone® HD plus Placebo; Fluzone® High-Dose (HD) plus MK-1406

MK-1406 Injection COMBINATION_PRODUCT

MK-1406 liquid for injection administered subcutaneously

Also known as: CD388

Fluzone® High-Dose (HD) plus MK-1406

Eligibility Criteria

• Age: 18 Years - 49 Years
• Sex: all
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64)

You may qualify if:

• 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

You may not qualify if:

• Has previously been diagnosed with Guillain-Barre Syndrome following a previous influenza vaccination.

Sponsors & Collaborators

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

Study Sites (4)

California Clinical Trials Medical Group (CCTMG) managed by Parexel

Glendale, California, 91206, United States

Location

Parexel ( Site 0001)

Glendale, California, 91206, United States

Location

Parexel ( Site 0002)

Baltimore, Maryland, 21225, United States

Location

Parexel International - EPCU Baltimore

Baltimore, Maryland, 21225, United States

Location

MeSH Terms

Conditions

Influenza, Human

Interventions

Influenza Vaccines

Condition Hierarchy (Ancestors)

Respiratory Tract Infections; Infections; Orthomyxoviridae Infections; RNA Virus Infections; Virus Diseases; Respiratory Tract Diseases

Intervention Hierarchy (Ancestors)

Viral Vaccines; Vaccines; Biological Products; Complex Mixtures

Study Officials

• Medical Director

  Merck Sharp & Dohme LLC

  STUDY DIRECTOR

Study Design

• Study Type: interventional
• Phase: phase 1
• Allocation: RANDOMIZED
• Masking: QUADRUPLE
• Who Masked: PARTICIPANT, CARE PROVIDER, INVESTIGATOR, OUTCOMES ASSESSOR
• Purpose: PREVENTION
• Intervention Model: PARALLEL
• Sponsor Type: INDUSTRY
• Responsible Party: SPONSOR

Study Record Dates

• First Submitted: December 10, 2025
• First Posted: December 23, 2025
• Study Start: November 8, 2025
• Primary Completion (Estimated): December 31, 2026
• Study Completion (Estimated): December 31, 2026
• Last Updated: August 19, 2026

Record last verified: 2026-08

Data Sharing

• IPD Sharing: Will share

Locations

US (4)