Adults over the age of 30 only catch flu about twice a decade, a new study suggests.
Flu-like illness can be caused by many pathogens, making it difficult to assess how often people are infected by influenza.
Researchers analysed blood samples from volunteers in Southern China, looking at antibody levels against nine different influenza strains that circulated from 1968 to 2009.
They found that while children get flu on average every other year, flu infections become less frequent as people progress through childhood and early adulthood. From the age of 30 onwards, flu infections tend to occur at a steady rate of about two per decade.
Dr Adam Kucharski, who worked on the study at Imperial College London before moving to the London School of Hygiene & Tropical Medicine, said: “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. Also, 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.”