A recent study by researchers at Imperial College London showed that adults older than 30 contract the flu only twice every decade.
A team of researchers from the college analyzed blood samples from Southern China to determine the levels of antibodies for nine different strains of influenza that existed from 1968 until 2009.
Children appear to contract the flu every other year; but as people age, the frequency of flu infections become less often. From approximately age 30 and older, the flu rates showed a rate of approximately two contractions every 10 years.
"For adults, we found that influenza infection is actually much less common than some people think,” Dr. Steven Riley, the senior author of the study and a scientist at the college's Medical Research Council Centre for Outbreak Analysis and Modelling, said. “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."
Many kinds of pathogens cause flu-like illnesses, which makes it impossible to tell exactly how frequently people contract influenza without a blood test.
"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,” Adam Kucharski, from Imperial College London and the London School of Hygiene & Tropical Medicine, said. “These symptoms could sometimes be caused by common cold viruses, such as rhinovirus or coronavirus. Also, some people might not realize 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."
Studying how the immune system works enables scientists to develop better treatments for illnesses.
"What we've done in this study is to analyse how a person's immunity builds up over a lifetime of flu infections,” Kucharski 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."