Asymptomatic spread of flu is not proved

BMJ 2005; 331 doi: https://doi.org/10.1136/bmj.331.7525.1145 (Published 10 November 2005) Cite this as: BMJ 2005;331:1145
  1. Ronald Eccles (eccles{at}cardiff.ac.uk), director
  1. Common Cold Centre, Cardiff University, Cardiff CF10 3US

    EDITOR—Macfarlane and Lim state that infection control of pandemic flu would be challenging because “unlike SARS, flu is highly infectious before patients develop definite symptoms.”1

    What is the evidence for asymptomatic transmission of flu? Fraser et al describe a model for the spread of flu, but it is only a mathematical model for transmission and the paper does not provide any convincing references to support asymptomatic spread.2 Older work by R B Couch in the 1970s states that viral shedding may sometimes occur before clinically significant symptoms, but viral shedding detected by nasal swabs cannot be equated to transmission of infection. Infected airway mucus must be exchanged for flu to transmit from one person to another.

    Rhinorrhoea, coughing, and sneezing are important factors in facilitating aerosol and fomites transmission of infection, and I doubt that any clinically significant exchange of airway mucus can occur from asymptomatic cases of flu.


    • Competing interests None declared.


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