(CNN) In 2009, two flu vaccines manufactured by the same company did not equally protect against the circulating H1N1 flu virus. Vaccine effectiveness for any given season varies, in part, based on how well the vaccine matches the circulating virus strains. The cells that produced this essential interferon are dendritic cells, noted Collin, who was not involved in the research. Two decades later, Jonas Salk and Thomas Francis developed the first flu vaccine. Although progress has been made over time refining the flu vaccine, the holy grail remains a "universal" vaccine.

Source: CNN March 22, 2017 18:00 UTC