Princeton-based researchers developed a model that revealed that the emerging class of cross-protective, or "universal," vaccines will allow for better flu prevention by limiting the influenza virus' ability to spread and mutate. For the above figure, the researchers simulated the effect of a basic cross-protective vaccine on a flu pandemic based on the proportion of the population that received it. They created three groups based on how much the vaccine reduced their contagiousness: by 25 (red), 50 (blue) and 75 (green) percent, respectively. At its most effective, the vaccine resulted in strong "herd immunity," or a general resistance to the virus despite only a part of the population receiving the vaccine. The percentage of the population infected with influenza dropped to zero even though only 70 percent of the population had vaccine protection.