The researchteam, including Dr. Adam Kucharski of the London School Hygiene and Tropical Medicine in the UK, reached its findings by analyzing blood antibody levels against nine different influenza A H3N2 strains that circulated in southern China between 1968 and 2009.
In the background of the study, the researchers explain that in response to flu viruses, the immune system produces antibodies that target proteins present on the surface of the virus. While these proteins change as the virus evolves, the antibodies that circulate in the blood remember previously encountered strains.
"However," the authors note, "the specific factors that shape the immune response of a single human to a particular strain are little understood because individual infections and the development of immunity over a lifetime in that person are rarely observed directly."
By analyzing the blood samples of Chinese children and adults, Dr. Kucharski and colleagues were able to identify the specific influenza strains with which an individual had been infected over the 40-year period, as well as the antibody response to each strain.
The researchers created a mathematical model that allowed them to estimate the frequency of flu infection by age between 1968 and 2009, as well as the way in which immunity to different flu strains changes over the course of a lifetime.
Flu strains we encounter in early life 'evoke stronger immune responses'
The team estimated flu transmission is much more frequent among children - they are likely to catch the virus every other year. From the age of 30, however, adults are only likely to contract flu twice a decade.
"For adults, we found that influenza infection is actually much less common than some people think," says senior author Dr. Steven Riley, of the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London in the UK. "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."
In addition, the researchers found that the strains of influenza virus we encounter early in life can activate stronger immune responses than the strains we come across later in life.
"Our results suggest that 'antigenic seniority,' whereby strains encountered earlier in life gain more 'senior' positions in the immune response, and short-lived cross-reactivity between different strains are important components of the immune response and, therefore, could shape the evolution and emergence of influenza viruses," they explain.
The team says their findings may help increase understanding of how immunity influences the evolution of flu viruses, paving the way for models that could predict how flu viruses will change.
In addition, the researchers say that understanding how previously contracted flu strains affect immunity could help scientists create more effective flu vaccines.
Dr. Kucharski adds:
"What we've done in this study is to analyze how a person's immunity builds up over a lifetime of flu infections. 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."