The typical adult over the age of 30 can expect to come down with the flu about twice every 10 years, a lower incidence than most people think, researchers behind a British study say.

In comparison, children typically catch influenza around every 2 years, they say.

"For adults, we found that influenza infection is actually much less common than some people think," says study author Dr. Steven Riley of Imperial College London.

"In childhood and adolescence, [flu is] much more common, possibly because we mix more with other people," he says.

Occasions of flu infections are less frequent as people advance through childhood and into early adulthood, researchers report in the journal PLoS Biology.

That may be because strains of influenza virus encountered earlier in life evoke stronger immune responses than those we encounter later, they suggest.

In adults older than 30, the frequency of flu infection can vary depending on surrounding levels of flu and a person's vaccination history, Riley says.

The frequency of flu infection in populations worldwide has been the subject of considerable discussion, says researcher Adam Kucharski, who took part in the study at Imperial College before going on to the London School of Hygiene & 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," he says. Although people may believe they've come down with influenza, "symptoms could sometimes be caused by common cold viruses, such as rhinovirus or coronavirus," he notes.

However, sometimes people have had flu without realizing it, he says, because it can occasionally be milder than what people expect.

"Some people might not realize they had flu, but the infection will show up when a blood sample is subsequently tested," Kucharski says.

Such blood samples were analyzed for the study, focusing on influenza A (H3N2), a dominant strain of flu virus worldwide that infects, sickens and kills the most people.

Blood samples were tested against nine strains of the H3N2 virus circulating between 1968 and 2009.

They could track those strains because blood retains an immunological "memory" of the flu strains a person has been infected with in the past, the researchers explained.

The researchers said by comparing blood antibodies with the dates that each flu strain was known to have been in circulation, they were able to determine how often, on average, people really get the flu.