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The 2009 pandemic influenza A (H1N1) virus has several genetically unique features, and public health experts are concerned that this virus could cause millions of human infections during the upcoming “flu season.” To predict disease activity related to the 2009 H1N1 virus, authorities need information about preexisting immunity to this pathogen in the population.
Toward this end, CDC investigators tested stored sera from individuals of various ages who lived in the U.S. or Europe and who participated in vaccine trials or had serum samples collected anonymously for blood donation or other purposes. They used a microneutralization assay, a hemagglutination-inhibition assay, or both, to measure levels of cross-reactive neutralizing antibodies …