Adults over the age of 30 only catch influenza about twice a decade, according to latest international research findings.
While children get influenza on average every other year, flu infections become less frequent as people progress through childhood and early adulthood, said the authors of a study.
“For adults, we found that influenza infection is actually much less common than some people think”
Researchers, including three from the UK, analysed blood samples from 151 volunteers in Southern China. They looked at antibody levels against nine different influenza strains that circulated from 1968 to 2009.
They found that, from the age of 30 onwards, flu infections tended to occur at a steady rate of about two per decade.
Study author Dr Adam Kucharski, from the London School of Hygiene and 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,” he noted.
“Also, some people might not realise they had flu, but the infection will show up when a blood sample is subsequently tested,” he added.
Co-author Dr Steven Riley, who is based at Imperial College London, said the findings showed that flu was “actually much less common than people think” in adults.
“In childhood and adolescence, it’s much more common, possibly because we mix more with other people,” he said. “The exact frequency of infection will vary depending on background levels of flu and vaccination.”
However, the researchers acknowledged that their findings were based on a relatively small sample from one region of the world. “It would, therefore, be helpful to test similar models of serodynamics against observed titres in other populations,” they wrote in the journal PLOS Biology.
The researchers also developed a mathematical model of how human immunity to flu changes over a lifetime as people encounter different strains of the virus.
They hope the work will help scientists consider how immunity to historical strains could influence the way vaccines work and how effective they will be.
Dr Kucharski said the study represented the first time that a group’s infection history had been reconstructed from modern-day blood samples.
“What we’ve done in this study is to analyse how a person’s immunity builds up over a lifetime of flu infections,” he said.
“This information 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,” he added.
Funding for the research came from a partnership including the Medical Research Council, the Wellcome Trust, Imperial College London, Public Health England and several US institutions.