Adults get flu just once every five years, new research finds

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According to a new study, flu is much less common in adults than people think and those of age 30 and above catch it only once every five years.

The findings of the study, published in the journal PLOS Biology, also reveal that children are much more prone to catching flu and they get it every other year on an average. As people progress from childhood to early adulthood, the frequency of flu infections decline and 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.”

For their study, researchers carried out a field study in China and analysed blood samples from volunteers 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.