The quadrivalent flu vaccine is becoming more available this year. The newer vaccine protects against four strains of flu: influenza A H1N1 virus, influenza A H3N2 virus and two influenza B virus strains. This is gaining preference over the trivalent vaccine that only covers three strains: two influenza A virus strains and one B virus strain. Find out how Saint Louis University’s Center for Vaccine Development played a role in the new vaccine.