Original Antigenic Sin and Pandemic (H1N1) 2009

Original Antigenic Sin and Pandemic (H1N1) 2009To the Editor: While pandemic (H1N1) 2009 was in its earliest stages, age distribution data indicated surprisingly few cases among persons >65 years of age.The initial assumption was that few persons >65 years of age had yet to be exposed.However, as more data became available from Mexico, Australia, and the United States, the age distribution pattern persisted (1).This observation raised the question about whether older persons were protected from infection with an infl uenza virus A (H1N1) strain acquired many years ago.Indeed, data from the Centers for Disease Control and Prevention showed that approximately two thirds of older persons have evidence of immunity to pandemic (H1N1) 2009 virus.In 1960, Thomas Francis proposed the hypothesis of original antigenic sin, a phenomenon whereby a person who as a child was fi rst exposed to a specifi c infl uenza virus A would, throughout life, mount an immune response to the virus of childhood, even when exposed to other antigenically dissimilar infl uenza viruses.In effect, the original antibody response generated by the immune system against a specifi c infl uenza viral strain was hypothesized to have colored all future responses to infl uenza (2).Serologic responses of humans and other mammals have supported this theory.A new hemagglutinin (HA) subtype emerged in 1918 that was responsible for the pandemic that year.Through 1956, the strain evolved, accumulating mutations.In an era before infl uenza viruses were subtyped was performed, the original 1918 infl uenza virus A (H1N1) was dubbed a swine strain, whereas the virus of the 1930s was known as infl uenza A. However, the amount of drift accrued by 1947