title: The seroprevalence of pandemic influenza H1N1 (2009) virus in China.
abstract: BACKGROUND
  Mainland China experienced pandemic influenza H1N1 (2009) virus (pH1N1) with peak activity during November-December 2009. To understand the geographic extent, risk factors, and attack rate of pH1N1 infection in China we conducted a nationwide serological survey to determine the prevalence of antibodies to pH1N1.
  
  
  RESULTS
  Stored serum samples (n = 2,379) collected during 2006-2008 were used to estimate baseline serum reactogenicity to pH1N1. In January 2010, we used a multistage-stratified random sampling method to select 50,111 subjects who met eligibility criteria and collected serum samples and administered a standardized questionnaire. Antibody response to pH1N1 was measured using haemagglutination inhibition (HI) assay and the weighted seroprevalence was calculated using the Taylor series linearization method. Multivariable logistic regression analyses were used to examine risk factors for pH1N1 seropositivity. Baseline seroprevalence of pH1N1 antibody (HI titer ≥40) was 1.2%. The weighted seroprevalence of pH1N1 among the Chinese population was 21.5%(vaccinated: 62.0%; unvaccinated: 17.1%). Among unvaccinated participants, those aged 6-15 years (32.9%) and 16-24 years (30.3%) had higher seroprevalence compared with participants aged 25-59 years (10.7%) and ≥60 years (9.9%, P<0.0001). Children in kindergarten and students had higher odds of seropositivity than children in family care (OR: 1.36 and 2.05, respectively). We estimated that 207.7 million individuals (15.9%) experienced pH1N1 infection in China.
  
  
  CONCLUSIONS
  The Chinese population had low pre-existing immunity to pH1N1 and experienced a relatively high attack rate in 2009 of this virus. We recommend routine control measures such as vaccination to reduce transmission and spread of seasonal and pandemic influenza viruses.
authors.affiliation: State Key Laboratory for Molecular Virology and Genetic Engineering, National Institute for Viral Infectious Disease Control and Prevention, Chinese Center for Disease Control and Prevention, Beijing, China.
authors.forename: Cuiling
authors.initials: C
authors.lastname: Xu
authors.forename: Tian
authors.initials: T
authors.lastname: Bai
authors.forename: A Danielle
authors.initials: AD
authors.lastname: Iuliano
authors.forename: Min
authors.initials: M
authors.lastname: Wang
authors.forename: Lei
authors.initials: L
authors.lastname: Yang
authors.forename: Leying
authors.initials: L
authors.lastname: Wen
authors.forename: Yuhong
authors.initials: Y
authors.lastname: Zeng
authors.forename: Xiaodan
authors.initials: X
authors.lastname: Li
authors.forename: Tao
authors.initials: T
authors.lastname: Chen
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Ying
authors.initials: Y
authors.lastname: Hu
authors.forename: Limei
authors.initials: L
authors.lastname: Yang
authors.forename: Zi
authors.initials: Z
authors.lastname: Li
authors.forename: Shumei
authors.initials: S
authors.lastname: Zou
authors.forename: Dexin
authors.initials: D
authors.lastname: Li
authors.forename: Shiwen
authors.initials: S
authors.lastname: Wang
authors.forename: Zijian
authors.initials: Z
authors.lastname: Feng
authors.forename: Yanping
authors.initials: Y
authors.lastname: Zhang
authors.forename: Hongjie
authors.initials: H
authors.lastname: Yu
authors.forename: Weizhong
authors.initials: W
authors.lastname: Yang
authors.forename: Yu
authors.initials: Y
authors.lastname: Wang
authors.forename: Marc-Alain
authors.initials: MA
authors.lastname: Widdowson
authors.forename: Yuelong
authors.initials: Y
authors.lastname: Shu
mesh_terms: D000293:Adolescent; D000328:Adult; D000914:Antibodies, Viral / Q000097:blood / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D002681:China / Q000453:epidemiology; D003429:Cross Reactions; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology / Q000302:isolation & purification*; D007252:Influenza Vaccines / Q000008:administration & dosage; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control / Q000821:virology*; D008875:Middle Aged; D016036:Seroepidemiologic Studies; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral; D007252:Influenza Vaccines