If you are reaching for your handkerchief it might be too late to stop the spread of flu, research suggests.

Flu and cold viruses are known to be carried in mucus droplets that spray out when a person coughs or sneezes. But the latest research indicates that flu can be transmitted before any symptoms show.

The findings, from a study of ferrets, support earlier research suggesting that viral particles can be expelled into the air through normal breathing.

sneezing flu spread

Research suggests people with flu have no symptoms

Lead researcher Professor Wendy Barclay, from 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."

Ferrets are often used in flu research because they are susceptible to the same virus strains as humans, and show similar symptoms.

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  • Vitamin C

    "Research has found no evidence that vitamin C prevents colds," says Dr Hasmukh Joshi, vice-chair of the Royal College of GPs. In 2007, the authors of a review of 30 trials involving 11,000 people concluded that, "regular ingestion of vitamin C has no effect on common cold incidence in the ordinary population". A daily dose of vitamin C did slightly reduce the length and severity of colds. When it comes to flu, one person in three believes that taking vitamin C can cure the flu virus. It can't. "Studies found that vitamin C offers a very, very limited benefit," says Dr Joshi. "I wouldn't recommend it." Information from <a href="http://www.nhs.uk/Livewell/coldsandflu/Pages/Preventionandcure.aspx" target="_hplink">NHS Choices</a>.

  • Echinacea

    The root, seeds and other parts of echinacea plants are used in herbal remedies that many people believe protect them against colds. There have been a number of studies into echinacea's effect, but no firm conclusions. A review of trials involving echinacea showed that, compared with people who didn't take echinacea, those who did were about 30% less likely to get a cold. However, the studies had varying results and used different preparations of echinacea. It's not known how these compare with the echinacea in shops. This review also showed that echinacea did not reduce the length of a cold when taken on its own. "There is a belief that echinacea aids the immune system, but a survey of studies in 2005 showed that it did not," says Dr Joshi. "I wouldn't recommend that it helps, but if people believe it, they can take it. There's no harm in it." Information from <a href="http://www.nhs.uk/Livewell/coldsandflu/Pages/Preventionandcure.aspx" target="_hplink">NHS Choices</a>.

  • Zinc

    There is some evidence that taking zinc lozenges as soon as cold symptoms appear may reduce how long a cold lasts. However, some trials have found no difference in the duration of colds in people who took zinc compared with those who did not. There has also been research into nasal sprays containing zinc. "Some people believe that the zinc lines the mucosa [the lining of the nose] and stops a cold virus attaching itself to the nose lining," says Dr Joshi. "Unfortunately, this has been found to be no more effective than a placebo." Information from <a href="http://www.nhs.uk/Livewell/coldsandflu/Pages/Preventionandcure.aspx" target="_hplink">NHS Choices</a>.

  • Getting cold or wet

    The only thing that can cause a cold or flu is a cold or flu virus. Getting cold and/or wet won't give you a cold. However, if you are already carrying the virus in your nose, it might allow symptoms to develop. A study at the Common Cold Centre in Cardiff found that people who chilled their feet in cold water for 20 minutes were twice as likely to develop a cold as those who didn't chill their feet. The authors suggest that this is because some people carry cold viruses without having symptoms. Getting chilled causes blood vessels in the nose to constrict, affecting the defences in the nose and making it easier for the virus to replicate. "Getting a cold from going out in the cold or after washing your hair is a myth," says Dr Joshi. "Colds are common. If the virus is already there and then you go out with wet hair and develop symptoms, it's common to think that is what caused it." Information from <a href="http://www.nhs.uk/Livewell/coldsandflu/Pages/Preventionandcure.aspx" target="_hplink">NHS Choices</a>.

  • So what does work?

    The flu vaccine can prevent you from catching flu. Apart from that, the best way to protect yourself from colds and flu is to have a healthy lifestyle. "Eat a healthy diet, take regular exercise and drink plenty of warm drinks in the winter months," says Dr Joshi. "The important thing to remember is that most people are going to catch a cold in winter anyway, because there is no effective cure for cold viruses." Information from <a href="http://www.nhs.uk/Livewell/coldsandflu/Pages/Preventionandcure.aspx" target="_hplink">NHS Choices</a>.

The new study, reported in the online journal PLoS ONE, is the first to investigate non-symptomatic flu transmission in an animal.

Ferrets with flu were placed close to healthy animals for a short period of time at different stages after infection.

Transmission occurred before the flu carriers displayed their first symptom: fever. The virus passed between animals which were kept both in the same cage and in adjacent cages.

The strain used in the research was the same one that caused the 2009 swine flu pandemic which killed almost 300,000 people worldwide.

Ferrets were able to pass flu onto their neighbours just 24 hours after being infected themselves, the scientists found.

Animals did not show signs of fever until 45 hours after infection and began sneezing after 48 hours.

In the late stages of infection, after five or six days, the virus was transmitted much less frequently. The researchers believe this suggests people could return to work or school soon after symptoms subside with little risk of infecting others.

Co-author Dr Kim Roberts, 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."