title: Prevalence of seropositivity to pandemic influenza A/H1N1 virus in the United States following the 2009 pandemic.
abstract: BACKGROUND
  2009 pandemic influenza A/H1N1 (A(H1N1)pdm09) was first detected in the United States in April 2009 and resulted in a global pandemic. We conducted a serologic survey to estimate the cumulative incidence of A(H1N1)pdm09 through the end of 2009 when pandemic activity had waned in the United States.
  
  
  METHODS
  We conducted a pair of cross sectional serologic surveys before and after the spring/fall waves of the pandemic for evidence of seropositivity (titer ≥40) using the hemagglutination inhibition (HI) assay. We tested a baseline sample of 1,142 serum specimens from the 2007-2008 National Health and Nutrition Examination Survey (NHANES), and 2,759 serum specimens submitted for routine screening to clinical diagnostic laboratories from ten representative sites.
  
  
  RESULTS
  The age-adjusted prevalence of seropositivity to A(H1N1)pdm09 by year-end 2009 was 36.9% (95%CI: 31.7-42.2%). After adjusting for baseline cross-reactive antibody, pandemic vaccination coverage and the sensitivity/specificity of the HI assay, we estimate that 20.2% (95%CI: 10.1-28.3%) of the population was infected with A(H1N1)pdm09 by December 2009, including 53.3% (95%CI: 39.0-67.1%) of children aged 5-17 years.
  
  
  CONCLUSIONS
  By December 2009, approximately one-fifth of the US population, or 61.9 million persons, may have been infected with A(H1N1)pdm09, including around half of school-aged children.
authors.affiliation: Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia, USA. creed1@cdc.gov
authors.forename: Carrie
authors.initials: C
authors.lastname: Reed
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Katz
authors.forename: Kathy
authors.initials: K
authors.lastname: Hancock
authors.forename: Amanda
authors.initials: A
authors.lastname: Balish
authors.forename: Alicia M
authors.initials: AM
authors.lastname: Fry
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000914:Antibodies, Viral / Q000097:blood; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology / Q000276:immunology* / Q000517:prevention & control; D008875:Middle Aged; D058873:Pandemics; D015995:Prevalence; D012680:Sensitivity and Specificity; D016036:Seroepidemiologic Studies; D014481:United States / Q000453:epidemiology; D014611:Vaccination; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral