title: Tracking social contact networks with online respondent-driven detection: who recruits whom?
abstract: BACKGROUND
  Transmission of respiratory pathogens in a population depends on the contact network patterns of individuals. To accurately understand and explain epidemic behaviour information on contact networks is required, but only limited empirical data is available. Online respondent-driven detection can provide relevant epidemiological data on numbers of contact persons and dynamics of contacts between pairs of individuals. We aimed to analyse contact networks with respect to sociodemographic and geographical characteristics, vaccine-induced immunity and self-reported symptoms.
  
  
  METHODS
  In 2014, volunteers from two large participatory surveillance panels in the Netherlands and Belgium were invited for a survey. Participants were asked to record numbers of contacts at different locations and self-reported influenza-like-illness symptoms, and to invite 4 individuals they had met face to face in the preceding 2 weeks. We calculated correlations between linked individuals to investigate mixing patterns.
  
  
  RESULTS
  In total 1560 individuals completed the survey who reported in total 30591 contact persons; 488 recruiter-recruit pairs were analysed. Recruitment was assortative by age, education, household size, influenza vaccination status and sentiments, indicating that participants tended to recruit contact persons similar to themselves. We also found assortative recruitment by symptoms, reaffirming our objective of sampling contact persons whom a participant may infect or by whom a participant may get infected in case of an outbreak. Recruitment was random by sex and numbers of contact persons. Relationships between pairs were influenced by the spatial distribution of peer recruitment.
  
  
  CONCLUSIONS
  Although complex mechanisms influence online peer recruitment, the observed statistical relationships reflected the observed contact network patterns in the general population relevant for the transmission of respiratory pathogens. This provides useful and innovative input for predictive epidemic models relying on network information.
authors.affiliation: Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands. M.L.Stein-2@umcutrecht.nl.|Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. M.L.Stein-2@umcutrecht.nl.
authors.forename: Mart L
authors.initials: ML
authors.lastname: Stein
authors.affiliation: Department of Methodology and Statistics, Faculty of Social and Behavioural Sciences, University Utrecht, Utrecht, The Netherlands. P.G.M.vanderHeijden@uu.nl.|Southampton Statistical Sciences Research Institute, University of Southampton, Southampton, UK. P.G.M.vanderHeijden@uu.nl.
authors.forename: Peter G M
authors.initials: PG
authors.lastname: van der Heijden
authors.affiliation: Department of Sociology, Faculty of Social and Behavioural Sciences, University Utrecht, Utrecht, The Netherlands. V.Buskens@uu.nl.
authors.forename: Vincent
authors.initials: V
authors.lastname: Buskens
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. Jim.van.Steenbergen@rivm.nl.|Centre of Infectious Diseases, Leiden University Medical Centre, Leiden, The Netherlands. Jim.van.Steenbergen@rivm.nl.
authors.forename: Jim E
authors.initials: JE
authors.lastname: van Steenbergen
authors.affiliation: Department of Public Health Sciences-Global Health, Karolinska Institutet, Stockholm, Sweden. Linus.Bengtsson@ki.se.|Flowminder Foundation, Stockholm, Sweden. Linus.Bengtsson@ki.se.
authors.forename: Linus
authors.initials: L
authors.lastname: Bengtsson
authors.affiliation: Science in Action BV, Amsterdam, The Netherlands. Carl@science-in-action.nl.
authors.forename: Carl E
authors.initials: CE
authors.lastname: Koppeschaar
authors.affiliation: Department of Public Health Sciences-Global Health, Karolinska Institutet, Stockholm, Sweden. Anna.Thorson@ki.se.
authors.forename: Anna
authors.initials: A
authors.lastname: Thorson
authors.affiliation: Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands. M.E.E.Kretzschmar@umcutrecht.nl.|Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. M.E.E.Kretzschmar@umcutrecht.nl.
authors.forename: Mirjam E E
authors.initials: ME
authors.lastname: Kretzschmar
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001530:Belgium; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D007223:Infant; D007251:Influenza, Human / Q000453:epidemiology / Q000209:etiology; D020407:Internet; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D057566:Self Report; D012919:Social Behavior; D012959:Socioeconomic Factors; D011795:Surveys and Questionnaires; D055815:Young Adult