title: Viral shedding and clinical illness in naturally acquired influenza virus infections.
abstract: BACKGROUND
  Volunteer challenge studies have provided detailed data on viral shedding from the respiratory tract before and through the course of experimental influenza virus infection. There are no comparable quantitative data to our knowledge on naturally acquired infections.
  
  
  METHODS
  In a community-based study in Hong Kong in 2008, we followed up initially healthy individuals to quantify trends in viral shedding on the basis of cultures and reverse-transcription polymerase chain reaction (RT-PCR) through the course of illness associated with seasonal influenza A and B virus infection.
  
  
  RESULTS
  Trends in symptom scores more closely matched changes in molecular viral loads measured with RT-PCR for influenza A than for influenza B. For influenza A virus infections, the replicating viral loads determined with cultures decreased to undetectable levels earlier after illness onset than did molecular viral loads. Most viral shedding occurred during the first 2-3 days after illness onset, and we estimated that 1%-8% of infectiousness occurs prior to illness onset. Only 14% of infections with detectable shedding at RT-PCR were asymptomatic, and viral shedding was low in these cases.
  
  
  CONCLUSIONS
  Our results suggest that "silent spreaders" (ie, individuals who are infectious while asymptomatic or presymptomatic) may be less important in the spread of influenza epidemics than previously thought.
authors.affiliation: Infectious Disease Epidemiology Group, School of Public Health, University of Hong Kong, Hong Kong, China.
authors.forename: Lincoln L H
authors.initials: LL
authors.lastname: Lau
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Cowling
authors.forename: Vicky J
authors.initials: VJ
authors.lastname: Fang
authors.forename: Kwok-Hung
authors.initials: KH
authors.lastname: Chan
authors.forename: Eric H Y
authors.initials: EH
authors.lastname: Lau
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.forename: Calvin K Y
authors.initials: CK
authors.lastname: Cheng
authors.forename: Peter M
authors.initials: PM
authors.lastname: Houck
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Uyeki
authors.forename: J S Malik
authors.initials: JS
authors.lastname: Peiris
authors.forename: Gabriel M
authors.initials: GM
authors.lastname: Leung
mesh_terms: D017714:Community-Acquired Infections; D006801:Humans; D009980:Influenza A virus / Q000502:physiology*; D009981:Influenza B virus / Q000502:physiology*; D007251:Influenza, Human / Q000473:pathology* / Q000821:virology*; D013997:Time Factors; D019562:Viral Load; D017201:Virus Shedding