title: Association between antibody titers and protection against influenza virus infection within households.
abstract: BACKGROUND
  Previous studies have established that antibody titer measured by the hemagglutination-inhibiting (HAI) assay is correlated with protection against influenza virus infection, with an HAI titer of 1:40 generally associated with 50% protection.
  
  
  METHODS
  We recruited index cases with confirmed influenza virus infection from outpatient clinics, and followed up their household contacts for 7-10 days to identify secondary infections. Serum samples collected from a subset of household contacts were tested by HAI and microneutralization (MN) assays against prevalent influenza viruses. We analyzed the data using an individual hazard-based transmission model that adjusted for age and vaccination history.
  
  
  RESULTS
  Compared to a reference group with antibody titers <1:10, we found that HAI titers of 1:40 against influenza A(H1N1) and A(H3N2) were associated with 31% (95% confidence interval [CI], 13%-46%) and 31% (CI, 1%-53%) protection against polymerase chain reaction (PCR)-confirmed A(H1N1) and A(H3N2) virus infection, respectively, while an MN titer of 1:40 against A(H3N2) was associated with 49% (95% CI, 7%-81%) protection against PCR-confirmed A(H3N2) virus infection.
  
  
  CONCLUSIONS
  An HAI titer of 1:40 was associated with substantially less than 50% protection against PCR-confirmed influenza virus infection within households, perhaps because of exposures of greater duration or intensity in that confined setting.
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Tim K
authors.initials: TK
authors.lastname: Tsang
authors.affiliation: Mathematical Modelling of Infectious Diseases Unit, Institut Pasteur, Paris, France.
authors.forename: Simon
authors.initials: S
authors.lastname: Cauchemez
authors.affiliation: Division of Public Health Laboratory Sciences, School of Public Health Centre of Influenza Research.
authors.forename: Ranawaka A P M
authors.initials: RA
authors.lastname: Perera
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Guy
authors.initials: G
authors.lastname: Freeman
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Vicky J
authors.initials: VJ
authors.lastname: Fang
authors.affiliation: Division of Community Medicine and Public Health Practice, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, China.
authors.forename: Dennis K M
authors.initials: DK
authors.lastname: Ip
authors.affiliation: Division of Community Medicine and Public Health Practice, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, China.
authors.forename: Gabriel M
authors.initials: GM
authors.lastname: Leung
authors.affiliation: Division of Public Health Laboratory Sciences, School of Public Health Centre of Influenza Research.
authors.forename: Joseph Sriyal
authors.initials: JS
authors.lastname: Malik Peiris
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Cowling
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D005191:Family Characteristics; D005192:Family Health; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D007223:Infant; D007251:Influenza, Human / Q000276:immunology* / Q000517:prevention & control* / Q000635:transmission; D008297:Male; D008875:Middle Aged; D009500:Neutralization Tests; D009975:Orthomyxoviridae / Q000235:genetics / Q000276:immunology* / Q000302:isolation & purification; D016133:Polymerase Chain Reaction; D012367:RNA, Viral / Q000235:genetics; D055815:Young Adult
chemical_list: D000914:Antibodies, Viral; D012367:RNA, Viral
keywords: antibody; immunity; influenza; transmission