This illustration depicts the antigenic shifts that occurred during the 20th century. The H1 subtype that circulated starting 1918 was replaced by an H2 subtype in 1957. Of note: when the H1 subtype reappeared in 1977, young persons were disproportionately affected as persons born before 1957 would have had previous exposure to H1 virus.
The H5, H7, and H9 subtypes depicted have not caused sustained person-to-person transmission and will be discussed further later on.