For many, flu season is an ominous event, guaranteeing (at best) yet another flu shot or (at worst) several days incapacitated with the virus. Now, a new study from Imperial College London puts into perspective just how prevalent the flu is: The average adult over 30 is likely to contract the virus only twice per decade.
Chances are, if you’re feeling sick it’s something else.
“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,” said Dr Adam Kucharski, who worked on the study.
The study involved blood samples from participants in Southern China. They measured antibody levels corresponding with nine different influenza strains identified between 1968 and 2009.
They found that children are just as flu-prone as they seem: Young children contract the virus every year, on average. The odds of catching the disease decrease through adolescence into early adulthood, eventually stabilizing at a rate of about every five years after age 30.
There are several possible reasons for this. One posited by researchers is that young people tend to congregate in much larger groups than adults (think overcrowded classrooms) and are less likely to worry about hygiene. It’s also possible that the free will that comes with adulthood spurs more people to get flu vaccines every year. The researchers did note that background and vaccination history affected the likelihood of infection.
There’s also another factor at play, one that could help scientists understand how the body responds to pathogens over time. When infected by a virus, the body responds by sending antibodies to attack proteins on the virus’ surface. Even after the virus is gone, the body retains the antibodies, which effectively have “memory” of the virus they eliminated.
A model produced by the researchers confirmed that strains of the virus contracted earlier in life prompt a stronger immune response than those contracted later. This could be the body recognizing that it’s younger self is more vulnerable to the virus, but it could also suggest that older bodies have more existing tools at their disposal to fight off infections.
Either way, the findings will help doctors understand how viruses evolve over time in response to immunity, a pressing concern in the medical field.
“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,” said Dr. Kucharski.