title: Individual Correlates of Infectivity of Influenza A Virus Infections in Households.
abstract: Identifying individual correlates of infectivity of influenza virus is important for disease control and prevention. Viral shedding is used as a proxy measure of infectivity in many studies. However, the evidence for this is limited.
  
  
  
  In a detailed study of influenza virus transmission within households in 2008-12, we recruited index cases with confirmed influenza infection from outpatient clinics, and followed up their household contacts for 7-10 days to identify secondary infections. We used individual-based hazard models to characterize the relationship between individual viral shedding and individual infectivity.
  
  
  
  We analyzed 386 households with 1147 household contacts. Index cases were separated into 3 groups according to their estimated level of viral shedding at symptom onset. We did not find a statistically significant association of virus shedding with transmission. Index cases in medium and higher viral shedding groups were estimated to have 21% (95% CI: -29%, 113%) and 44% (CI: -16%, 167%) higher infectivity, compared with those in the lower viral shedding group.
  
  
  
  Individual viral load measured by RT-PCR in the nose and throat was at most weakly correlated with individual infectivity in households. Other correlates of infectivity should be examined in future studies.
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, 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: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, 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: Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Kwok-Hung
authors.initials: KH
authors.lastname: Chan
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Dennis K M
authors.initials: DK
authors.lastname: Ip
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Gabriel M
authors.initials: GM
authors.lastname: Leung
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.|Centre of Influenza Research, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: J S Malik
authors.initials: JS
authors.lastname: Peiris
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, 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.identifier: 0000-0002-6297-7154
authors.initials: BJ
authors.lastname: Cowling
authors.affiliation: Mathematical Modelling of Infectious Diseases Unit, Institut Pasteur, Paris, France.
authors.forename: Simon
authors.initials: S
authors.lastname: Cauchemez
mesh_terms: D000293:Adolescent; D000328:Adult; D005191:Family Characteristics; D005260:Female; D006723:Hong Kong; D006801:Humans; D009980:Influenza A virus / Q000472:pathogenicity*; D007251:Influenza, Human / Q000821:virology*; D008297:Male; D008875:Middle Aged; D014774:Virulence; D017201:Virus Shedding