title: [Herd immunity against new influenza A (H1N1) in pre-vaccinated residents aged over 5 years in Beijing].
abstract: OBJECTIVE
  To explore the herd immunity against influenza A (H1N1) in pre-vaccinated residents aged over 5 years, and therefore to provide data for vaccination policies in high risk populations.
  
  
  METHODS
  From October to December 2009, Beijing CDC conducted a serum survey of the novel influenza A (H1N1) in the local residents, stratified in 10 age groups between 5 years to over 60 years, without H1N1 vaccination history and disease history. Hemagglutination inhibition (HI) assays were performed at Beijing CDC. Statistical significance was determined with geometric mean titer (GMT).
  
  
  RESULTS
  3499 serum samples were tested for HI antibody. The average level of HI antibody was 1:8.03, and 11.06% (387/3499) were sero-positive (HI antibody level ≥ 1:40). In the group aged from 5 to 19 years, the level of HI antibody and the sero-positive rate were higher (HI antibody > 1:8.9, sero-positive rate > 12%).
  
  
  CONCLUSIONS
  The antibody levels in different groups were affected by age specific morbidity, and the higher antibody level of the school-age group was correlated with higher disease intensity in this population. The data showed that the herd immunity in Beijing was under the optimal level, but influenza A (H1N1) would probably become prevalent in the short coming future.
authors.affiliation: Department of Immunization and Prevention, Beijing Center for Disease Prevention and Control, Beijing 100013, China.
authors.forename: Li
authors.initials: L
authors.lastname: Lu
authors.forename: Dong-lei
authors.initials: DL
authors.lastname: Liu
authors.forename: Tie-gang
authors.initials: TG
authors.lastname: Zhang
authors.forename: Meng
authors.initials: M
authors.lastname: Chen
authors.forename: Zhu-jiazi
authors.initials: ZJ
authors.lastname: Zhang
authors.forename: Xiao-li
authors.initials: XL
authors.lastname: Wang
authors.forename: Zhen
authors.initials: Z
authors.lastname: Yang
authors.forename: Xing-huo
authors.initials: XH
authors.lastname: Pang
authors.forename: Ying
authors.initials: Y
authors.lastname: Deng
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000914:Antibodies, Viral / Q000097:blood* / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D002681:China; D005260:Female; D006801:Humans; D032521:Immunity, Herd; D053118:Influenza A Virus, H1N1 Subtype; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology; D007251:Influenza, Human / Q000276:immunology / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral; D007252:Influenza Vaccines