Schematic of the H2N2 vaccine trial. Participants were placed into four groups separated by exposure status and vaccination platform: naive participants (born after H2N2 viruses ended circulation in 1968) who were primed with H2 DNA-plasmid-based vaccine (group 1) or the multivalent H2-F nanoparticle (group 2) and pre-exposed participants (born before 1968) who were first vaccinated with the DNA-plasmid-based vaccine (group 3) or the H2-F nanoparticle (group 4). All groups received secondary vaccinations with H2-F. Individual participants are notated by -1, -2, and -3 for a total ofn= 3 per group.
