The purpose of this phase 1/2 study is to investigate the safety and immunogenicity of different doses (high, medium and low) of a second generation structurally designed (SD2) H5 messenger ribonucleic acid (mRNA) vaccine against pandemic H5 influenza virus (pandemic flu H5 hemagglutinin (HA) mRNA SD2) in healthy younger and older adults. The study will aim to identify the appropriate dose for further clinical development of a potential pandemic response vaccine. The study also includes an extension phase for one of the 3 dose levels of the pandemic flu H5 HA mRNA SD2 vaccine to collect additional safety and the immunogenicity data for this specific dose of the vaccine. During this Extension Phase, an additional 480 participants will be randomized according to a 1:1 ratio and stratified by age (≥ 18 to \< 65 years and ≥ 65 years) to receive either the low dose of the pandemic flu H5 HA mRNA DS2 vaccine (Group 1) or placebo (Group 4). This extension will enhance the safety database and improve precision of the immunogenicity results for the selected dose while preserving the original study design integrity. The study duration per participant will be approximately 13 months. There will be two injections of placebo or pandemic flu H5 mRNA vaccine 21 days apart at high, medium and low doses. Study visits/contact include: 7 study visits and 1 telephone call. Vaccination visits (including blood samples) will occur at Day 01 and Day 22. Short-term follow-up visits (including blood samples) will occur 8 and 21 days after each injection. Participants will be also followed up (including blood samples) at 3 and 6 months after 2nd injection, and at 12 months after 2nd injection for safety.
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Presence of immediate adverse events (AEs)
Timeframe: Within 30 minutes after each/any injection
Presence of solicited injection site reactions
Timeframe: Through 7 days after each/any injection
Presence of solicited systemic reactions
Timeframe: Through 7 days after each/any injection
Presence of unsolicited AEs
Timeframe: Through 21 days after the first injection through 28 days after the second injection
Presence of medically attended adverse events (MAAEs)
Timeframe: Through 180 days after the last injection
Presence of adverse events of special interest (AESIs)
Timeframe: Throughout the study, approximately 13 months
Presence of serious adverse events (SAEs)
Timeframe: Throughout the study, approximately 13 months
Presence of out-of-range biological test results (including shift from baseline values)
Timeframe: Through a maximum of 8 days after each injection