title: Construction of the influenza A virus transmission tree in a college-based population: co-transmission and interactions between influenza A viruses.
abstract: BACKGROUND
  Co-infection of different influenza A viruses is known to occur but how viruses interact within co-infection remains unknown. An outbreak in a college campus during the 2009 pandemic involved two subtypes of influenza A: persons infected with pandemic A/H1N1; persons infected with seasonal A/H3N2 viruses; and persons infected with both at the same time (co-infection). This provides data to analyse the possible interaction between influenza A viruses within co-infection.
  
  
  METHODS
  We extend a statistical inference method designed for outbreaks caused by one virus to that caused by two viruses. The method uses knowledge of which subtype each case is infected with (and whether they were co-infected), contact information and symptom onset date of each case in the influenza outbreak. We then apply it to construct the most likely transmission tree during the outbreak in the college campus.
  
  
  RESULTS
  Analysis of the constructed transmission tree shows that the simultaneous presence of the two influenza viruses increases the infectivity and the transmissibility of A/H1N1 virus but whether it changes the infectivity of A/H3N2 is unclear. The estimation also shows that co-transmission of both subtypes from co-infection is low and therefore co-infection cannot be sustained on its own.
  
  
  CONCLUSIONS
  This study suggests that influenza A viruses within co-infected patients can interact in some ways rather than transmit independently, and this can enhance the spread of influenza A virus infection.
authors.affiliation: Centre for Infectious Disease Surveillance and Control, Public Health England, 61 Colindale Avenue, London, NW9 5EQ, UK. xu-sheng.zhang@phe.gov.uk.|Medical Research Council Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College School of Public Health, Norfolk Place, London, W2 1PG, UK. xu-sheng.zhang@phe.gov.uk.
authors.forename: Xu-Sheng
authors.initials: XS
authors.lastname: Zhang
authors.affiliation: Centre for Infectious Disease Surveillance and Control, Public Health England, 61 Colindale Avenue, London, NW9 5EQ, UK.|Medical Research Council Biostatistics Unit, University Forvie Site, Robinson Way, Cambridge, CB2 0SR, UK.
authors.forename: Daniela
authors.initials: D
authors.lastname: De Angelis
mesh_terms: D000465:Algorithms; D060085:Coinfection / Q000453:epidemiology; D004196:Disease Outbreaks; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification* / Q000472:pathogenicity; D053122:Influenza A Virus, H3N2 Subtype / Q000302:isolation & purification* / Q000472:pathogenicity; D007251:Influenza, Human / Q000453:epidemiology / Q000635:transmission* / Q000821:virology
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