Oxford University Press, Clinical Infectious Diseases, 3(54), p. 311-317, 2011
DOI: 10.1093/cid/cir809
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Preexisting antibody, responses to seasonal and pandemic 2009 vaccine, a low infection-to-illness ratio, and shortened illnesses indicated preexisting immunity to pandemic H1N1 virus that caused the 2009 epidemic to be mild in healthy adults.