Flu is much less common in adults than you might think

Scientists claim adults over 30 might get it twice a decade

People are often confused by flu-like symptoms (Tom Merton/Getty Creative)

Adults over the age of 30 typically get flu about twice every 10 years whereas children contract the infection once every two years on average, a study has revealed.

As children grow up they begin to build up immunity to flu viruses and their rate of infection slows down until it reaches a plateau at about 30, according to a study of blood samples from volunteers in China that contain antibodies to influenza strains circulating between 1968 and 2009.

People may think they suffer more frequently from flu infections but in fact they are often confused by flu-like symptoms caused by a range of other respiratory viruses, said Adam Kucharski of the London School of Hygiene and Tropical Medicine.

“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 or coronavirus,” Dr Kucharski said.

“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,” he said.

Steven Riley of Imperial College, senior author of the study published in the on-line journal PLOS Biology, said: “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 exact frequency of infection will vary depending on background levels of flu and vaccination.”