title: A Large Proportion of the Mexican Population Remained Susceptible to A(H1N1)pdm09 Infection One Year after the Emergence of 2009 Influenza Pandemic.
abstract: BACKGROUND
  The 2009 H1N1 influenza pandemic initially affected Mexico from April 2009 to July 2010. By August 2010, a fourth of the population had received the monovalent vaccine against the pandemic virus (A(H1N1)pdm09). To assess the proportion of the Mexican population who remained potentially susceptible to infection throughout the summer of 2010, we estimated the population seroprevalence to A(H1N1)pdm09 in a serosurvey of blood donors.
  
  
  METHODS
  We evaluated baseline cross-reactivity to the pandemic strain and set the threshold for seropositivity using pre-pandemic (2005-2008) stored serum samples and sera from confirmed A(H1N1)pdm09 infected individuals. Between June and September 2010, a convenience sample serosurvey of adult blood donors, children, and adolescents was conducted in six states of Mexico. Sera were tested by the microneutralization (MN) and hemagglutination inhibition (HI) assays, and regarded seropositive if antibody titers were equal or exceeded 1:40 for MN and 1:20 for HI. Age-standardized seroprevalence were calculated using the 2010 National Census population.
  
  
  RESULTS
  Sera from 1,484 individuals were analyzed; 1,363 (92%) were blood donors, and 121 (8%) children or adolescents aged ≤19 years. Mean age (standard deviation) was 31.4 (11.5) years, and 276 (19%) were women. A total of 516 (35%) participants declared history of influenza vaccination after April 2009. The age-standardized seroprevalence to A(H1N1)pdm09 was 48% by the MN and 41% by the HI assays, respectively. The youngest quintile, aged 1 to 22 years, had the highest the seroprevalence; 61% (95% confidence interval [CI]: 56, 66%) for MN, and 56% (95% CI: 51, 62%) for HI.
  
  
  CONCLUSIONS
  Despite high transmission of A(H1N1)pdm09 observed immediately after its emergence and extensive vaccination, over a half of the Mexican population remained potentially susceptible to A(H1N1)pdm09 infection. Subsequent influenza seasons with high transmission of A(H1N1)pdm09, as 2011-2012 and 2013-2014, are compatible with these findings.
authors.affiliation: Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Vic
authors.initials: V
authors.lastname: Veguilla
authors.affiliation: Dirección General Adjunta de Epidemiología, Secretaría de Salud, Mexico City, México.
authors.forename: Hugo
authors.initials: H
authors.lastname: López-Gatell
authors.affiliation: Laboratorio de Virus Respiratorios, Departamento de Virología, Instituto de Diagnóstico y Referencia Epidemiológicos, Mexico City, México.
authors.forename: Irma
authors.initials: I
authors.lastname: López-Martínez
authors.affiliation: Laboratorio de Virus Respiratorios, Departamento de Virología, Instituto de Diagnóstico y Referencia Epidemiológicos, Mexico City, México.
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Aparicio-Antonio
authors.affiliation: Laboratorio de Virus Respiratorios, Departamento de Virología, Instituto de Diagnóstico y Referencia Epidemiológicos, Mexico City, México.
authors.forename: Gisela
authors.initials: G
authors.lastname: Barrera-Badillo
authors.affiliation: Centro Nacional de la Transfusión Sanguínea, Secretaría de Salud, Mexico City, México.
authors.forename: Julieta
authors.initials: J
authors.lastname: Rojo-Medina
authors.affiliation: Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Felicia Liaini
authors.initials: FL
authors.lastname: Gross
authors.affiliation: Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Stacie N
authors.initials: SN
authors.lastname: Jefferson
authors.affiliation: Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Katz
authors.affiliation: Subsecretaría de Prevención y Promoción de la Salud, Secretaría de Salud, Mexico City, México.
authors.forename: Mauricio
authors.initials: M
authors.lastname: Hernández-Ávila
authors.affiliation: Dirección General Adjunta del Instituto de Diagnóstico y Referencia Epidemiológicos, Mexico City, México.
authors.forename: Celia M
authors.initials: CM
authors.lastname: Alpuche-Aranda
mesh_terms: D000328:Adult; D000914:Antibodies, Viral / Q000276:immunology; D003429:Cross Reactions / Q000276:immunology; D005260:Female; D006385:Hemagglutination Inhibition Tests / Q000379:methods; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology; D007252:Influenza Vaccines / Q000276:immunology; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology; D008297:Male; D017058:Mexican Americans; D008800:Mexico / Q000453:epidemiology; D016036:Seroepidemiologic Studies; D014611:Vaccination / Q000379:methods
chemical_list: D000914:Antibodies, Viral; D007252:Influenza Vaccines