Research at Imperial College London examining influenza transmission in ferrets suggests that the virus can be transmitted before the symptoms could appear. If the finding applies to humans than it means that people pass on flu to others before they know they're infected.
The research was supported by the National Institute for Health Research (NIHR) Imperial Biomedical Research Centre.
Knowing if people are infectious before they have symptoms is important to help authorities plan for an epidemic, but is has been difficult to establish this from data collected during outbreaks. Previous research using mathematical models estimated that most flu transmission occurs after the onset of symptoms, but some happens earlier.
Ferrets with flu were put in contact with uninfected ferrets for short periods at different stages after infection. Transmission occurred before the first symptom, fever, appeared, both when the ferrets were in the same cage and when they were in adjacent cages.
Professor Wendy Barclay, the study's lead author from the Department of Medicine at Imperial College London, said: "This result has important implications for pandemic planning strategies. It means that the spread of flu is very difficult to control, even with self-diagnosis and measures such as temperature screens at airports. It also means that doctors and nurses who don't get the flu jab are putting their patients at risk because they might pass on an infection when they don't know they're infected."
The flu strain used in the study was from the 2009 swine flu pandemic, which killed almost 300,000 people worldwide.
In the late stages of infection, after five or six days, flu was transmitted much less frequently, suggesting that people can return to work or school soon after symptoms subside with little risk of passing flu on to others.
The first author, Dr Kim Roberts, who is now based at Trinity College Dublin, said: "Ferrets are the best model available for studying flu transmission, but we have to be cautious about interpreting the results in humans. We only used a small number of animals in the study, so we can't say what proportion of transmission happens before symptoms occur. It probably varies depending on the flu strain."
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