London: Adults over the age of 30 only catch flu about once every five years, researchers have calculated.

While children get flu on average every other year, flu infections become less frequent as people progress through childhood and early adulthood, the findings showed.

From the age of 30 onwards, flu infections tend to occur at a steady rate of about two per decade.

“For adults, we found that influenza infection is actually much less common than some people think,” said senior author of the study Steven Riley from at Imperial College London.

“In childhood and adolescence, it is much more common, possibly because we mix more with other people,” Riley explained.

Researchers analysed blood samples from volunteers in China, looking at antibody levels against nine different influenza strains that circulated from 1968 to 2009.

“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,” said Adam Kucharski, who worked on the study at Imperial College London before moving to the London School of Hygiene & Tropical Medicine.

“These symptoms could sometimes be caused by common cold viruses, such as rhinovirus or coronavirus. Also, some people might not realise they had flu, but the infection will show up when a blood sample is subsequently tested,” Kucharski noted.

Flu-like illness can be caused by many pathogens, making it difficult to assess how often people are infected by influenza.

In addition to estimating the frequency of flu infection, the researchers developed a mathematical model of how our immunity to flu changes over a lifetime as we encounter different strains of the virus.

The model supported evidence from other studies that the strains of influenza virus we encounter earlier in life evoke stronger immune responses than those we meet later.
The study appeared in the journal PLOS Biology.