Flu is much less common in adults than some people think, a new study says.

Children catch the flu once every two years but flu infections become rarer as people move from childhood to early adulthood.  By the age after 30, flu occurs only at a steady rate of about once every five years.

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The immune system produces antibodies to fight flu viruses by targeting proteins on the virus surface. These proteins change as the virus evolve but the body keeps antibodies in the blood that have a memory for strains it encountered before.

There have been many debates in the medical community as to how frequent people get the flu compared to flu-like illnesses cause by something else, said Dr. Adam Kucharski from the London School of Hygiene and Tropical Medicine.

Dr. Kucharski said the symptoms people get could only be caused by common cold viruses. Some people might also not realize they had flu as the infection sometimes shows up only when a blood sample is subsequently tested.

For the study, an international team of researchers analyzed blood samples from 151 volunteers from China between the age of seven and 81.

Gathering the data, which hasn't been done before, could help experts better understand who's at risk of the infection, how often and how far the disease spreads to the community.

Researchers looked at the nine main flu strains that circulated around the world from 1968 to 2009. All the strains are types of influenza A (H3N2) virus.

The findings suggest that for adults, typically from the age 30 onwards, influenza infection is actually less common compared to children and adolescents.

The researchers pointed out the findings may not apply to other populations and that the exact frequency of infection will vary depending on background levels and vaccination.