Adults get flu 'about once every five years'

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Adults catch real flu about once every five years, scientists calculate, based on a field study in China.

Although many of us will feel ill more often than this, other flu-like infections are to usually blame, the international team says.

The scientists tested blood samples from 151 volunteers aged between seven and 81, to gauge how often flu infections strike.

A similar study in the UK will test if the findings apply to British people.

Gathering this sort of lifespan data - which the researchers say has not really been done before - should help experts better understand who is at risk of infection, and how often, as well as how far the disease spreads through communities.

The study, in the journal PLoS Biology, looked at nine main strains of flu known to have been circulating around the globe between 1968 and 2009.

These were all types of influenza A (H3N2) virus.

Blood clues

The researchers, from Imperial College London and institutes in the US and China, checked the volunteers' blood for the presence of antibodies - to reveal whether they had ever been infected with the viruses, and how often.

They found that while the children got flu on average every other year, flu infections became less frequent with age.

From the age of 30 onwards, flu infections tended to occur at a steady rate of about two per decade in the people that they studied.

The researchers point out that their findings may not apply to other populations but conceivably could.

Dr Steven Riley, senior author of the study, from the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial, said: "The exact frequency of infection will vary depending on background levels of flu and vaccination."

He said the flu that they had studied included strains that had spread widely and quickly around the world, including Europe and the US.

"For adults, we found that influenza infection is actually less common than some people think," he said.

"In childhood and adolescence, it's more common, possibly because children mix more frequently with other people."

Prof Ron Eccles, of the Common Cold Centre at Cardiff University, said: "This is an interesting study.

"There is much confusion between colds and flu and influenza."

He said some infections were also silent - they might only show up if you went looking for them in blood.

Cold or flu?

Colds and flu are caused by viruses, but it can be tricky to tell these diseases apart based on just subjective symptoms.

Both can cause a temperature and sore throat, plus coughs and sneezes - which is how they spread from person to person.

What one person considers to be flu, another may call a heavy head cold, for example.

With a common cold, the symptoms tend to come on gradually, are usually milder and affect mostly the nose and the throat.

Flu is a more severe illness that normally makes the individual want to take to their bed. The symptoms include aches and pains as well as a blocked or runny nose.

Flu is unpleasant but not threatening for most fit and healthy adults, but it can cause serious complications in the elderly and people with other chronic health conditions, such as asthma.

This is why experts recommend at-risk groups get a flu jab every year to protect themselves.

In the UK this year, flu has been circulating at its highest level for three years. The virus is a strain of influenza type A (H3N2).

This year's choice of seasonal flu vaccine sparked criticism after studies found the jab stopped only three out of every 100 vaccinated people from developing symptoms.

The flu virus is constantly mutating, which makes it difficult to predict which strain to vaccinate against each year.