While you may feel like you get the flu every winter, in reality if you are over the age of thirty it is likely that you catch the flu only about twice each decade. This is one finding of a study published today in the journal PLOS Biology. In the study, a multi-national research team based in the United Kingdom, United States and China focused on the long-lived antibody response to influenza A virus over the lifetime of an individual.



Flu-like symptoms can be caused by pathogens other than influenza virus, so it can be difficult to be sure how often a particular individual genuinely has the flu. 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.”



In order to determine how often people get the flu over the course of their lifetime, the research team analysed blood samples from volunteers in Southern China. They measured antibody titres to nine different strains of the influenza A (H3N2) virus which circulated between 1968 and 2009. In this way, they obtained data on frequency of flu virus infection and the ages at which infection occurred.

While new antibodies are produced against evolving influenza virus strains, we also keep antibodies with memory for previous strains we have encountered. The results of the study indicated that ‘antigenic seniority’ is an important component of the immune response against influenza virus, whereby we mount a stronger immune response against strains we encounter early in life and these strains therefore occupy a more ‘senior position’ in our immune response. Short-lived cross-reactivity between strains is also an important immune mechanism. Both antigenic seniority and short-lived cross-reactivity could therefore be important influences on viral evolution and emergence.



With regard to frequency of infection, the results of the study showed that children catch flu on average every second year. This frequency declines through childhood and early adulthood, until we succumb to flu only about twice every ten years after the age of thirty. Senior author Dr Steven Riley, of the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London, said: “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. The exact frequency of infection will vary depending on background levels of flu and vaccination.”



This study is unusual in that individual responses to viral infections and how their immunity develops over their lifetime is less commonly examined than whole population effects. The results should help understanding of how individual immunity to historical strains affects immunity to current strains and the influence this has on vaccine effectiveness. Dr Kucharski said: “What we’ve done in this study is to analyse 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.”