The study focuses on influenza, a viral infection that causes fever, cough, and body aches. The investigational product is a mRNA-based seasonal influenza vaccine that is given as a single injection into the muscle. For comparison, two standard flu vaccines, Efluelda TIV‑HD and another trivalent influenza vaccine, are also used.
The purpose of the study is to determine whether the new vaccine produces an immune response that is not worse than the response seen with the existing vaccines and to assess its safety. Immunogenicity, which means the ability of the vaccine to stimulate the body’s defenses, will be measured by looking at antibodies against the virus’s surface proteins called HA and NA. These antibodies are measured with a test called HI (hemagglutination inhibition). Reactogenicity, or the short‑term side effects such as soreness at the injection site, fever, or muscle aches, will be recorded for a week after vaccination, and any other health problems will be followed for several months. Some participants may receive a placebo instead of the active vaccine.
Participants will receive one dose of the assigned product, stay for a short observation period, and then return for brief visits or phone calls at about one week, one month, and up to six months to collect safety information and blood samples for antibody testing. The study is designed to be double‑blind, meaning neither the participants nor the staff who assess outcomes will know which product was given, to keep the results unbiased.
Belgium 
Germany 
