(University of Pennsylvania School of Medicine) A universal flu vaccine that protects people against most influenza strains is one step closer to reality, with a study from the Perelman School of Medicine at the University of Pennsylvania. The candidate vaccine, described in Nature Communications this week, elicited a strong antibody response to a structure on the surface of flu viruses, called the hemagglutinin (HA) stalk. It protected mice from infection by various flu strains.