There is hemagglutinin on the viral surface. The viral hemagglutinins have at least 18 types, but these types are broken into two main influenza A categories. For example, one of the two main categories includes human H1, H2, and avian H5 viruses while the other major category includes human H3 and avian H7 viruses. Researchers in 2016 at UCLA and the University of Arizona discovered that if you were exposed to one of these groups as a child, you had a much better chance of being protected against other viruses in that same group or category later in life. For example, if you are exposed to H2 as a child and then later in life to H2 or H5 viruses, you may have as high as a 75% chance of protection against those H2 and/or H5 strains, but if you are exposed to the other major category that included H3 or H7, you would be much more susceptible to these viral types. The reverse situation would be true if you were exposed as a child to H3 or H7 viruses. The researchers concluded that the immunological imprinting early in life helps determine the response (immune response) to these viral types or categories. Consequently, the first strain of flu that a person is exposed to in childhood likely determines that person's risk in the future for severity of the flu depending upon the exact category of the first viral strain that infects the child. The researchers hope to exploit these new findings in the development of new and more effective flu vaccines.