title: The first cases of 2009 pandemic influenza A (H1N1) virus infection in the United States: a serologic investigation demonstrating early transmission.
abstract: BACKGROUND
  The first two laboratory-confirmed cases of 2009 pandemic influenza A (H1N1) virus (H1N1pdm09) infection were detected in San Diego (SD) and Imperial County (IC) in southern California, April 2009.
  
  
  OBJECTIVE
  To describe H1N1pdm09 infections and transmission early in the 2009 H1N1 pandemic.
  
  
  METHODS
  We identified index case-patients from SD and IC with polymerase chain reaction (PCR)-confirmed H1N1pdm09 infections and investigated close contacts for a subset of case-patients from April 17-May 6, 2009. Acute and convalescent serum was collected. Serologic evidence for H1N1pdm09 infection was determined by microneutralization and hemagglutination inhibition assays.
  
  
  RESULTS
  Among 75 close contacts of seven index case-patients, three reported illness onset prior to patient A or B, including two patient B contacts and a third with no links to patient A or B. Among the 69 close contacts with serum collected >14 days after the onset of index case symptoms, 23 (33%) were seropositive for H1N1pdm09, and 8 (35%) had no fever, cough, or sore throat. Among 15 household contacts, 8 (53%) were seropositive for H1N1pdm09. The proportion of contacts seropositive for H1N1pdm09 was highest in persons aged 5-24 years (50%) and lowest in persons aged ≥ 50 years (13%) (P = 0·07).
  
  
  CONCLUSIONS
  By the end of April 2009, before H1N1pdm09 was circulating widely in the community, a third of persons with close contact to confirmed H1N1pdm09 cases had H1N1pdm09 infection in SD and IC. Three unrelated clusters during March 21-30 suggest that transmission of H1N1pdm09 had begun earlier in southern California.
authors.affiliation: Centers for Disease Control and Prevention, Atlanta, GA, USA. afry@cdc.gov
authors.forename: Alicia M
authors.initials: AM
authors.lastname: Fry
authors.forename: Kathy
authors.initials: K
authors.lastname: Hancock
authors.forename: Minal
authors.initials: M
authors.lastname: Patel
authors.forename: Matthew
authors.initials: M
authors.lastname: Gladden
authors.forename: Saumil
authors.initials: S
authors.lastname: Doshi
authors.forename: Dianna M
authors.initials: DM
authors.lastname: Blau
authors.forename: David
authors.initials: D
authors.lastname: Sugerman
authors.forename: Vic
authors.initials: V
authors.lastname: Veguilla
authors.forename: Xiuhua
authors.initials: X
authors.lastname: Lu
authors.forename: Heather
authors.initials: H
authors.lastname: Noland
authors.forename: Yaohui
authors.initials: Y
authors.lastname: Bai
authors.forename: Azarnoush
authors.initials: A
authors.lastname: Maroufi
authors.forename: Annie
authors.initials: A
authors.lastname: Kao
authors.forename: Paula
authors.initials: P
authors.lastname: Kriner
authors.forename: Karla
authors.initials: K
authors.lastname: Lopez
authors.forename: Michele
authors.initials: M
authors.lastname: Ginsberg
authors.forename: Seema
authors.initials: S
authors.lastname: Jain
authors.forename: Sonja J
authors.initials: SJ
authors.lastname: Olsen
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Katz
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002140:California / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000175:diagnosis* / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D008297:Male; D008875:Middle Aged; D009500:Neutralization Tests; D058873:Pandemics; D014481:United States / Q000453:epidemiology; D055815:Young Adult