It is those two viral proteins, the hemagglutinin and the neuraminidase, that define a flu strain, and scientists began naming strains by their hemagglutinin and neuraminidase proteins. A strain that swept the world in 1946, for example, was H1N1. The next time the flu virus underwent a major genetic change, creating a pandemic, was in 1956, with strain H2N2. The pandemic that arrived in 1968 involved a virus whose hemagglutinin had changed from the 1956 virus but whose neuraminidase had not. It was named H3N2.
