A new study has revealed that adults over the age of 30 only catch flu about twice a decade.
Flu-like illness can be caused by many pathogens, making it difficult to assess how often people are infected by influenza. The immune system responds to flu viruses by producing antibodies that specifically target proteins on the virus surface. These proteins can change as the virus evolves, but people keep antibodies in the blood that have a memory for strains they've encountered before.
Researchers, who analysed blood samples from volunteers in Southern China, 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.
Adam Kucharski of London School of Hygiene and Tropical Medicine said that 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.
Senior author Steven Riley said that for adults, they found that influenza infection is actually much less common than some people think. In childhood and adolescence, it's much more common, possibly because we mix more with other people. The exact frequency of infection will vary depending on background levels of flu and vaccination.
The findings will help understanding how the immunity in the population affects the evolution of flu viruses, and potentially make predictions about how the virus will change in the future. They could also help scientists consider how immunity to historical strains will influence the way vaccines work and how effective they will be.
The study appears in PLOS Biology suggests.
