Adults only get flu twice a decade on average, scientists have found, suggesting that most of the coughs and colds that keep millions of people off work every year are down to other bugs.
The findings will deepen understanding about how the disease spreads, who is most at risk and how to develop and deploy vaccines to combat it, said researchers who conducted the study.
"For adults, we 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," said Steven Riley of Imperial College London, who worked on the research.
The team analyzed blood samples from volunteers in southern China, looking at antibody levels against nine different flu strains that circulated from 1968 to 2009.
They found that while children get flu on average every other year, flu infections became less frequent with age.
"Flu-like illnesses" can often be caused by other viruses such as rhinoviruses and coronaviruses, the researchers said, making it tricky for people to know if they have real flu.
As well as estimating flu's frequency, the team, including researchers from Britain, the United States and China, developed a mathematical model of how immunity to flu changes over a lifetime as people encounter different virus strains.
The findings, published in the journal PLOS Biology, could help researchers and drugmakers predict how the virus will change in future and how immunity to historical strains influences the way vaccines work and how effective they will be.
"What we've done in this study is to analyze how a person's immunity builds up over a lifetime of flu infections," said Adam Kucharski, who worked on the study at Imperial before moving to the London School of Hygiene & Tropical Medicine.
"This ... helps us understand the susceptibility of the population as a whole and how easy it is for new seasonal strains to spread through the population."