title: Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature.
abstract: BACKGROUND
  The potential impact of an influenza pandemic can be assessed by calculating a set of transmissibility parameters, the most important being the reproduction number (R), which is defined as the average number of secondary cases generated per typical infectious case.
  
  
  METHODS
  We conducted a systematic review to summarize published estimates of R for pandemic or seasonal influenza and for novel influenza viruses (e.g. H5N1). We retained and summarized papers that estimated R for pandemic or seasonal influenza or for human infections with novel influenza viruses.
  
  
  RESULTS
  The search yielded 567 papers. Ninety-one papers were retained, and an additional twenty papers were identified from the references of the retained papers. Twenty-four studies reported 51 R values for the 1918 pandemic. The median R value for 1918 was 1.80 (interquartile range [IQR]: 1.47-2.27). Six studies reported seven 1957 pandemic R values. The median R value for 1957 was 1.65 (IQR: 1.53-1.70). Four studies reported seven 1968 pandemic R values. The median R value for 1968 was 1.80 (IQR: 1.56-1.85). Fifty-seven studies reported 78 2009 pandemic R values. The median R value for 2009 was 1.46 (IQR: 1.30-1.70) and was similar across the two waves of illness: 1.46 for the first wave and 1.48 for the second wave. Twenty-four studies reported 47 seasonal epidemic R values. The median R value for seasonal influenza was 1.28 (IQR: 1.19-1.37). Four studies reported six novel influenza R values. Four out of six R values were <1.
  
  
  CONCLUSIONS
  These R values represent the difference between epidemics that are controllable and cause moderate illness and those causing a significant number of illnesses and requiring intensive mitigation strategies to control. Continued monitoring of R during seasonal and novel influenza outbreaks is needed to document its variation before the next pandemic.
authors.affiliation: Epidemiology and Prevention Branch, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road NE MS A-32, Atlanta 30333, Georgia. mbiggerstaff@cdc.gov.
authors.forename: Matthew
authors.initials: M
authors.lastname: Biggerstaff
authors.forename: Simon
authors.initials: S
authors.lastname: Cauchemez
authors.forename: Carrie
authors.initials: C
authors.lastname: Reed
authors.forename: Manoj
authors.initials: M
authors.lastname: Gambhir
authors.forename: Lyn
authors.initials: L
authors.lastname: Finelli
mesh_terms: D000818:Animals; D006801:Humans; D009980:Influenza A virus / Q000145:classification / Q000502:physiology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D058873:Pandemics; D012621:Seasons; D015047:Zoonoses / Q000453:epidemiology* / Q000635:transmission / Q000821:virology