Seasonal Influenza

The purpose of this study is to evaluate safety, reactogenicity, and immunogenicity of mRNA-1010 in healthy children (9 to \<18 years of age).

Background: Vaccines help the body learn to fight infections. Some vaccines are combined with adjuvants, which are added substances that make vaccines work better. FluMos-v2 is an experimental flu vaccine; ALFQ is an experimental adjuvant. Objective: To test FluMos-v2, with and without the ALFQ adjuvant, in healthy adults. Eligibility: Healthy adults aged 18 to 50 years. They must have received at least one flu vaccine from the 2020-21 season through the 2024-25 flu season. They must also agree not to receive the licensed 2025-26 flu vaccine. Design: Participants will have 12 clinic visits over 15 months. Participants will be screened. They will have a physical exam and blood tests. On 2 visits, about 4 months apart, participants will receive a vaccination. The shots will be given into the muscle of the upper arm. They will get a follow-up call the day after each shot. They will keep a daily diary for 7 days; they will record their temperature and any other symptoms they feel after each shot. All clinic visits will include collection of blood, saliva, and nasal secretions. If participants develop flu symptoms (such as fever, runny nose, sore throat), they will be asked to come to the clinic. About 2 weeks after each vaccination, participants may opt to undergo apheresis: Blood will be taken from the body through a needle inserted into one arm. The blood will pass through a machine that separates out the white blood cells. The remaining blood will be returned to the body through a needle in the other arm.