Flu infection is less common than many people think, with adults over the age of 30 only catching it about twice a decade, a new study has found.

According to international researchers, many people may think they have flu, but actually have a flu-like illness, while others may not realise when they do have it.

The researchers analysed the blood samples of people living in China. The samples were assessed for antibody levels against nine different flu strains that circulated between 1968 and 2009.

They found that on average, children developed flu every other year. However, as people moved through childhood and into early adulthood, the illness became less frequent. In fact, from the age of 30, the illness occurred at a steady rate of around two times per decade.

"There's a lot of debate in the field as to how often people get flu, as opposed to flu-like illness caused by something else. These symptoms could sometimes be caused by common cold viruses, such as rhinovirus. Also, some people might not realise they had flu, but the infection will show up when a blood sample is subsequently tested.

"This is the first time anyone has reconstructed a group's history of infection from modern-day blood samples," explained Dr Adam Kucharski, formerly of Imperial College London.

The researchers from the UK, the US and China also found further evidence that strains of the flu virus that people develop earlier in life provoke a stronger immune response than strains they develop later.

"What we've done in this study is to analyse how a person's immunity builds up over a lifetime of flu infections. For adults, we found that influenza infection is actually much less common than some people think. In childhood and adolescence, it's much more common, possibly because we mix more with other people," the researchers said.

Details of these findings are published in the journal, PLOS Biology.

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