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Majority of flu patients are 'symptom free'


Less than a quarter of people infected in recent flu outbreaks actually showed any symptoms, UK researchers have claimed.

They said that almost one in five people were infected in both recent outbreaks of seasonal flu and the 2009 H1N1 influenza pandemic. But only 23% of these infections caused symptoms, they suggest.

Their study, published in the Lancet Respiratory Medicine, states that reported cases of flu are “the tip of the iceberg”. They examined the extent of flu burden in England over a five-year period.

“Reported cases of influenza represent the tip of a large clinical and subclinical iceberg”

Andrew Hayward

The Flu Watch study tracked more than 3,000 people across England during six flu seasons between 2006 and 2011.

People taking part in the study provided blood samples before and after each flu season, and were contacted weekly to see whether they had been suffering from any coughs, colds or flu-like illnesses. Anyone with such symptoms had a nasal swap to test for illness.

Flu 'symptom free' for most

The researchers found that on average, 18% of people who had not received a flu jab were infected with flu each season.

But most of the infections (77%) didn’t have any symptoms, they found, and only 17% of those infected felt ill enough to visit their GP.

The authors suggest that current surveillance systems greatly underestimate the rates of infection in society.

“Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention,” said lead author Dr Andrew Hayward from University College London.

“Most people don’t go to the doctor when they have flu. Even when they do consult they are often not recognised as having influenza,” he said.

“This study poses some big questions about how we should monitor the incidence of influenza”

Ronald Eccles

“Surveillance based on patients who consult greatly underestimates the number of community cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die.

“Information on the community burden is therefore critical to inform future control and prevention programmes,” he added.

Commenting on the study, Professor Ronald Eccles, director of Cardiff University’s Common Cold Centre, said: “This study poses some big questions about how we should monitor the incidence of influenza infections in the community.

Ronald Eccles

Ronald Eccles

“Influenza can be a killer, but what will surprise most people is that the majority of influenza infections do not cause any symptoms − not even a sniffle.”



Readers' comments (4)

  • michael stone

    Isn't it true with many infections, for the majority of those 'infected' to only suffer inconsequential [or even un-noticable] symptoms ?

    I suppose what is actually of more interest here, is whether the people in the community who have got 'symptom-free flu' can still pass the illness on, or whether these 'non-sufferers' are also non contagious ?

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  • Were the 17% with symptoms in the 18% of those not vaccinated?...or does it include some, or all, of those that were vaccinated?

    I think it also poses the question, is the human body coping with the flu virus, as it should, in those with who have no symptoms? If symptom-free people are identified, what are we expected to do with them - quarantine? I believe if we vaccinate the whole population we are heading for trouble, no wonder the little flu bug blighter mutates, it is to get ahead of the human race, and as we know, nature always wins.

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  • We are part of nature so to look at things as a war between humans and nature isn't necessarily helpful. But in battles against infectious diseases humans can win - look at smallpox, to cite just one example.

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  • michael stone

    Anonymous | 18-Mar-2014 2:31 pm

    I had the flu very bad twice when I was a kid.

    Perhaps I wasn't 'at war' with the flu - but the thing definitely wasn't on my side !!! It certainly felt, for about 24-48 hours (long time ago - exact period uncertain), as if it was making a serious attempt to kill me.

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